btrfs: qgroup: move WARN_ON() to the correct location.

In function qgroup_excl_accounting(), we need to WARN when
qg->excl is less than what we want to free, same to child
and parents. But currently, for parent qgroup, the WARN_ON()
is located after freeing qg->excl. It will WARN out even we
free it normally.

This patch move this WARN_ON() before freeing qg->excl.
Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Reviewed-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/qgroup.c

@@ -1431,9 +1431,8 @@ static int qgroup_excl_accounting(struct btrfs_fs_info *fs_info,
 		qgroup = u64_to_ptr(unode->aux);
 		qgroup->rfer += sign * oper->num_bytes;
 		qgroup->rfer_cmpr += sign * oper->num_bytes;
+		WARN_ON(sign < 0 && qgroup->excl < oper->num_bytes);
 		qgroup->excl += sign * oper->num_bytes;
-		if (sign < 0)
-			WARN_ON(qgroup->excl < oper->num_bytes);
 		qgroup->excl_cmpr += sign * oper->num_bytes;
 		qgroup_dirty(fs_info, qgroup);