ext4l: Extract uaccess.h declarations into their own file

Create include/linux/uaccess.h with stub implementations for user-space
access functions. In U-Boot there's no user/kernel separation, so these
are simple memory copies.

Includes:
- copy_from_user()
- copy_to_user()
- get_user() / put_user()
- access_ok()

Update compat.h to include uaccess.h and remove the duplicate
declaration.

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

lib/linux_compat.c

@@ -10,13 +10,6 @@ struct p_current cur = {
 };
 __maybe_unused struct p_current *current = &cur;
 
-unsigned long copy_from_user(void *dest, const void *src,
-		     unsigned long count)
-{
-	memcpy((void *)dest, (void *)src, count);
-	return 0;
-}
-
 void *kmalloc(size_t size, int flags)
 {
 	void *p;