btrfs: do not BUG_ON() on failure to reserve metadata for delayed item

At btrfs_insert_delayed_dir_index(), we don't expect the metadata
reservation for the delayed dir index item insertion to fail, because the
caller is supposed to have reserved 1 unit of metadata space for that.
All callers are able to deal with an error in case that happens, so there
is no need for something so drastic as a BUG_ON() in case of failure.
Instead just emit a warning, so that's easily noticed during development
(fstests in particular), and return the error to the caller.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/delayed-inode.c

@@ -1386,10 +1386,13 @@ int btrfs_insert_delayed_dir_index(struct btrfs_trans_handle *trans,
 
 	ret = btrfs_delayed_item_reserve_metadata(trans, dir->root, delayed_item);
 	/*
-	 * we have reserved enough space when we start a new transaction,
-	 * so reserving metadata failure is impossible
+	 * Space was reserved for a dir index item insertion when we started the
+	 * transaction, so getting a failure here should be impossible.
 	 */
-	BUG_ON(ret);
+	if (WARN_ON(ret)) {
+		btrfs_release_delayed_item(delayed_item);
+		goto release_node;
+	}
 
 	mutex_lock(&delayed_node->mutex);
 	ret = __btrfs_add_delayed_insertion_item(delayed_node, delayed_item);