linux: Add compatibility headers for ext4l

Add stub headers to support ext4l filesystem compilation:

New headers:
- bit_spinlock.h: bit-based spinlock stubs
- blkdev.h: block device structures and operations
- buffer_head.h: buffer head management with state functions
- crc32c.h: CRC32C checksum stub
- fs.h: filesystem structures (super_block, inode operations)
- journal-head.h: journal buffer head structure
- mutex.h: mutex stubs

Updates to existing headers:
- compat.h: add rcu_head callback structure
- jbd2.h: add wait.h and init.h includes for types
- sched.h: add current task stub
- workqueue.h: fix flush_work macro warning

These provide the minimal Linux kernel interfaces needed for
ext4 code to compile in U-Boot's single-threaded environment.

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

include/linux/bit_spinlock.h

@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * bit-based spin_lock()
+ *
+ * Minimal version for U-Boot ext4l - based on Linux 6.18
+ * U-Boot is single-threaded so these are simplified.
+ */
+#ifndef __LINUX_BIT_SPINLOCK_H
+#define __LINUX_BIT_SPINLOCK_H
+
+#include <linux/bitops.h>
+
+/*
+ * bit-based spin_lock() - U-Boot single-threaded version
+ */
+static inline void bit_spin_lock(int bitnum, unsigned long *addr)
+{
+	set_bit(bitnum, addr);
+}
+
+static inline int bit_spin_trylock(int bitnum, unsigned long *addr)
+{
+	if (test_bit(bitnum, addr))
+		return 0;
+	set_bit(bitnum, addr);
+	return 1;
+}
+
+static inline void bit_spin_unlock(int bitnum, unsigned long *addr)
+{
+	clear_bit(bitnum, addr);
+}
+
+static inline void __bit_spin_unlock(int bitnum, unsigned long *addr)
+{
+	clear_bit(bitnum, addr);
+}
+
+static inline int bit_spin_is_locked(int bitnum, unsigned long *addr)
+{
+	return test_bit(bitnum, addr);
+}
+
+#endif /* __LINUX_BIT_SPINLOCK_H */

include/linux/blkdev.h

@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Block device definitions
+ *
+ * Minimal version for U-Boot ext4l - based on Linux 6.18
+ */
+#ifndef _LINUX_BLKDEV_H
+#define _LINUX_BLKDEV_H
+
+#include <linux/types.h>
+
+struct block_device;
+struct gendisk;
+
+/* Block size helpers */
+#define bdev_logical_block_size(bdev)	512
+
+#endif /* _LINUX_BLKDEV_H */

include/linux/crc32c.h

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * CRC32C definitions
+ *
+ * Minimal version for U-Boot ext4l - based on Linux 6.18
+ */
+#ifndef _LINUX_CRC32C_H
+#define _LINUX_CRC32C_H
+
+#include <linux/types.h>
+#include <u-boot/crc.h>
+
+/* Use U-Boot's CRC32 implementation */
+static inline u32 crc32c(u32 crc, const void *address, unsigned int length)
+{
+	return crc32(crc, address, length);
+}
+
+#define crc32c_le(crc, p, len)	crc32c(crc, p, len)
+
+#endif /* _LINUX_CRC32C_H */

include/linux/fs.h

@@ -0,0 +1,83 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Filesystem definitions
+ *
+ * Minimal version for U-Boot ext4l - based on Linux 6.18
+ */
+#ifndef _LINUX_FS_H
+#define _LINUX_FS_H
+
+#include <linux/types.h>
+#include <linux/list.h>
+#include <linux/mutex.h>
+
+/* Forward declarations */
+struct inode;
+struct super_block;
+struct buffer_head;
+
+/* errseq_t - error sequence type */
+typedef u32 errseq_t;
+
+/* fmode_t - file mode type */
+typedef unsigned int fmode_t;
+
+/* File mode flags */
+#define FMODE_READ		((__force fmode_t)(1 << 0))
+#define FMODE_WRITE		((__force fmode_t)(1 << 1))
+#define FMODE_LSEEK		((__force fmode_t)(1 << 2))
+
+/* Buffer operations are in buffer_head.h */
+
+/* address_space - minimal stub */
+struct address_space {
+	struct inode *host;
+	errseq_t wb_err;	/* For jbd2 error tracking */
+};
+
+/* block_device - minimal stub */
+struct block_device {
+	struct address_space *bd_mapping;
+	void *bd_disk;
+};
+
+/* errseq functions - stubs */
+static inline int errseq_check(errseq_t *eseq, errseq_t since)
+{
+	return 0;
+}
+
+static inline int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)
+{
+	return 0;
+}
+
+/* file - minimal stub */
+struct file {
+	fmode_t f_mode;
+	struct inode *f_inode;
+};
+
+/* iattr - inode attributes for setattr */
+struct iattr {
+	unsigned int ia_valid;
+	umode_t ia_mode;
+	uid_t ia_uid;
+	gid_t ia_gid;
+	loff_t ia_size;
+};
+
+/* writeback_control - defined in linux/compat.h */
+
+/* fsnotify - stub */
+#define fsnotify_change(d, m)	do { } while (0)
+
+/* inode_init_once - stub */
+static inline void inode_init_once(struct inode *inode)
+{
+}
+
+/* S_ISDIR, etc. - already in linux/stat.h */
+#include <linux/stat.h>
+
+#endif /* _LINUX_FS_H */

include/linux/jbd2.h

@@ -29,6 +29,8 @@
 #include <linux/bit_spinlock.h>
 #include <linux/blkdev.h>
 #include <linux/crc32c.h>
+#include <linux/wait.h>
+#include <linux/init.h>
 #endif
 
 #define journal_oom_retry 1

include/linux/sched.h

@@ -14,6 +14,7 @@
 struct task_struct {
 	int pid;
 	char comm[16];
+	void *journal_info;	/* For jbd2 */
 };
 
 extern struct task_struct *current;

include/linux/workqueue.h

@@ -23,7 +23,7 @@ struct delayed_work {
 #define cancel_work_sync(work)			0
 #define cancel_delayed_work(work)		0
 #define cancel_delayed_work_sync(work)		0
-#define flush_work(work)			0
+#define flush_work(work)			({ (void)(work); 0; })
 #define flush_delayed_work(work)		0
 #define queue_work(wq, work)			0
 #define queue_delayed_work(wq, work, delay)	0