fs/xattr: Annotate struct simple_xattr with __counted_by

Add the __counted_by() compiler attribute to the flexible array member
'value' to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20260105122057.2347-2-thorsten.blum@linux.dev
Signed-off-by: Kees Cook <kees@kernel.org>

Author: toblux

include/linux/xattr.h

@@ -114,7 +114,7 @@ struct simple_xattr {
 	struct rb_node rb_node;
 	char *name;
 	size_t size;
-	char value[];
+	char value[] __counted_by(size);
 };
 
 void simple_xattrs_init(struct simple_xattrs *xattrs);