ext4l: Add stubs from various other linux headers

Add stub headers for various Linux kernel interfaces that ext4 code
expects:

- sched.h: scheduler stubs (task_struct, cond_resched, yield)
- wait.h: wait queue stubs
- rwsem.h: read-write semaphore stubs
- percpu_counter.h: percpu counter implementation (single-threaded)
- random.h: random number stubs
- quotaops.h: disk quota operation stubs
- part_stat.h: partition statistics stubs
- prefetch.h: prefetch operation stubs
- sort.h: sort wrapper using stdlib qsort
- swap.h: swap/memory management stubs

Update compat.h to include new headers and remove duplicate
definitions.

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

Series-to: concept
Cover-letter:
ext4l: Add Linux compatibility headers
This series extracts Linux kernel compatibility declarations from
include/linux/compat.h into their own header files, matching the
Linux kernel's organization. This makes it easier to port Linux
filesystem code to U-Boot and keeps the compatibility layer
maintainable.

The headers come from Linux v6.18

Headers added:
- export.h: EXPORT_SYMBOL macros
- stddef.h: sizeof_field() macro
- uaccess.h: copy_to/from_user stubs
- capability.h, cred.h, file.h, path.h, security.h, seq_file.h
- freezer.h: process freezer stubs
- slab.h, vmalloc.h: memory allocation
- module.h: kernel module stubs
- init.h: initcall macros
- kthread.h: kernel thread stubs
- timer.h, workqueue.h: timer and workqueue stubs
- sched.h, wait.h, rwsem.h: scheduler and synchronization
- percpu_counter.h, random.h, quotaops.h, part_stat.h, prefetch.h,
  sort.h, swap.h

All headers include appropriate copyright/author information from
the original Linux sources.
END
Series-links: 1:77
Series-version: 2

Signed-off-by: Simon Glass <simon.glass@canonical.com>

lib/linux_compat.c

@@ -5,10 +5,10 @@
 #include <asm/cache.h>
 #include <linux/compat.h>
 
-struct p_current cur = {
+struct task_struct cur = {
 	.pid = 1,
 };
-__maybe_unused struct p_current *current = &cur;
+__maybe_unused struct task_struct *current = &cur;
 
 void *kmalloc(size_t size, gfp_t flags)
 {