btrfs: replace the BUG_ON in btrfs_del_root_ref with proper error handling

I hit the BUG_ON() with generic/475 test case, and to my surprise, all
callers of btrfs_del_root_ref() are already aborting transaction, thus
there is not need for such BUG_ON(), just go to @out label and caller
will properly handle the error.

CC: stable@vger.kernel.org # 5.4+
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NQu Wenruo <wqu@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/root-tree.c

@@ -336,7 +336,8 @@ int btrfs_del_root_ref(struct btrfs_trans_handle *trans, u64 root_id,
 	key.offset = ref_id;
 again:
 	ret = btrfs_search_slot(trans, tree_root, &key, path, -1, 1);
-	BUG_ON(ret < 0);
+	if (ret < 0)
+		goto out;
 	if (ret == 0) {
 		leaf = path->nodes[0];
 		ref = btrfs_item_ptr(leaf, path->slots[0],