btrfs: qgroup: Fix a rebase bug which will cause qgroup double free

When rebasing my patchset, I forgot to pick up a cleanup patch to remove
old hotfix in 4.2 release.

Witouth the cleanup, it will screw up new qgroup reserve framework and
always cause minus reserved number.
Signed-off-by: NQu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/qgroup.c

@@ -1652,10 +1652,6 @@ static int qgroup_update_counters(struct btrfs_fs_info *fs_info,
 			}
 		}
 
-		/* For exclusive extent, free its reserved bytes too */
-		if (nr_old_roots == 0 && nr_new_roots == 1 &&
-		    cur_new_count == nr_new_roots)
-			qg->reserved -= num_bytes;
 		if (dirty)
 			qgroup_dirty(fs_info, qg);
 	}