ext4l: Add orphan.c and required support

Add orphan.c to the ext4l build and fix various build issues:

- Add atomic_inc/atomic_dec macros to ext4_uboot.h
- Add s_flags member to struct super_block and SB_RDONLY constant
- Add bdev_read_only() stub function
- Add kvfree() and kvmalloc_array() to linux/slab.h
- Add stub functions for orphan.c: ext4_reserve_inode_write,
  ext4_superblock_csum_set, ext4_mark_iloc_dirty, ext4_truncate,
  ext4_feature_set_ok, ext4_bread
- Fix orphan.c to use U-Boot include pattern
- Remove linux/atomic.h include from buffer_head.h (types provided
  by ext4_uboot.h)
- Add kunit/static_stub.h and linux/nospec.h stub headers
- Simplify seq_file.h macros

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

include/kunit/static_stub.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _KUNIT_STATIC_STUB_H
+#define _KUNIT_STATIC_STUB_H
+
+/*
+ * Stub header for U-Boot ext4l.
+ *
+ * KUnit static stubs are for kernel unit testing - not needed in U-Boot.
+ */
+
+#define KUNIT_STATIC_STUB_REDIRECT(func, args...)	do { } while (0)
+
+#endif /* _KUNIT_STATIC_STUB_H */