btrfs: avoid starting new transaction when cleaning qgroup during subvolume drop

At btrfs_qgroup_cleanup_dropped_subvolume() all we want to commit the
current transaction in order to have all the qgroup rfer/excl numbers up
to date. However we are using btrfs_start_transaction(), which joins the
current transaction if there is one that is not yet committing, but also
starts a new one if there is none or if the current one is already
committing (its state is >= TRANS_STATE_COMMIT_START). This later case
results in unnecessary IO, wasting time and a pointless rotation of the
backup roots in the super block.

So instead of using btrfs_start_transaction() followed by a
btrfs_commit_transaction(), use btrfs_commit_current_transaction() which
achieves our purpose and avoids starting and committing new transactions.

Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Filipe Manana <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: fdmanana

fs/btrfs/qgroup.c

@@ -1880,11 +1880,7 @@ int btrfs_qgroup_cleanup_dropped_subvolume(struct btrfs_fs_info *fs_info, u64 su
 	 * Commit current transaction to make sure all the rfer/excl numbers
 	 * get updated.
 	 */
-	trans = btrfs_start_transaction(fs_info->quota_root, 0);
-	if (IS_ERR(trans))
-		return PTR_ERR(trans);
-
-	ret = btrfs_commit_transaction(trans);
+	ret = btrfs_commit_current_transaction(fs_info->quota_root);
 	if (ret < 0)
 		return ret;