VFS: move BUG_ON test for symlink nd->depth after current->link_count test

This solves a serious VFS-level bug in nested_symlink (which was
rewritten from do_follow_link), and follows the order of depth tests
that existed before.

The bug triggers a BUG_ON in fs/namei.c:1381, when running racer with
symlink and rename ops.
Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>
Acked-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/namei.c

@@ -1378,12 +1378,12 @@ static inline int nested_symlink(struct path *path, struct nameidata *nd)
 {
 	int res;
 
-	BUG_ON(nd->depth >= MAX_NESTED_LINKS);
 	if (unlikely(current->link_count >= MAX_NESTED_LINKS)) {
 		path_put_conditional(path, nd);
 		path_put(&nd->path);
 		return -ELOOP;
 	}
+	BUG_ON(nd->depth >= MAX_NESTED_LINKS);
 
 	nd->depth++;
 	current->link_count++;