btrfs: do proper error handling in btrfs_insert_xattr_item

The space check in btrfs_insert_xattr_item is duplicated in it's caller
(do_setxattr) so we won't hit the BUG_ON. Continuing without any check
could be disasterous so turn it to a proper error handling.
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/dir-item.c

@@ -80,7 +80,8 @@ int btrfs_insert_xattr_item(struct btrfs_trans_handle *trans,
 	struct extent_buffer *leaf;
 	u32 data_size;
 
-	BUG_ON(name_len + data_len > BTRFS_MAX_XATTR_SIZE(root->fs_info));
+	if (name_len + data_len > BTRFS_MAX_XATTR_SIZE(root->fs_info))
+		return -ENOSPC;
 
 	key.objectid = objectid;
 	key.type = BTRFS_XATTR_ITEM_KEY;