Btrfs: fix mutex unlock without prior lock on space cache truncation

If the call to btrfs_truncate_inode_items() failed and we don't have a block
group, we were unlocking the cache_write_mutex without having locked it (we
do it only if we have a block group).

Fixes: 1bbc621 ("Btrfs: allow block group cache writeout
                      outside critical section in commit")

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/free-space-cache.c

@@ -231,13 +231,15 @@ int btrfs_truncate_free_space_cache(struct btrfs_root *root,
 {
 	int ret = 0;
 	struct btrfs_path *path = btrfs_alloc_path();
+	bool locked = false;
 
 	if (!path) {
 		ret = -ENOMEM;
 		goto fail;
 	}
 
 	if (block_group) {
+		locked = true;
 		mutex_lock(&trans->transaction->cache_write_mutex);
 		if (!list_empty(&block_group->io_list)) {
 			list_del_init(&block_group->io_list);
@@ -269,18 +271,14 @@ int btrfs_truncate_free_space_cache(struct btrfs_root *root,
 	 */
 	ret = btrfs_truncate_inode_items(trans, root, inode,
 					 0, BTRFS_EXTENT_DATA_KEY);
-	if (ret) {
-		mutex_unlock(&trans->transaction->cache_write_mutex);
-		btrfs_abort_transaction(trans, root, ret);
-		return ret;
-	}
+	if (ret)
+		goto fail;
 
 	ret = btrfs_update_inode(trans, root, inode);
 
-	if (block_group)
-		mutex_unlock(&trans->transaction->cache_write_mutex);
-
 fail:
+	if (locked)
+		mutex_unlock(&trans->transaction->cache_write_mutex);
 	if (ret)
 		btrfs_abort_transaction(trans, root, ret);