Btrfs: fix the qgroup reserved space is released prematurely

In start_transactio(), we will try to join the transaction again after
the current transaction is committed, so we should not release the
reserved space of the qgroup. Fix it.

Cc: Arne Jansen <sensille@gmx.net>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/transaction.c

@@ -383,7 +383,7 @@ again:
 	h->block_rsv = NULL;
 	h->orig_rsv = NULL;
 	h->aborted = 0;
-	h->qgroup_reserved = qgroup_reserved;
+	h->qgroup_reserved = 0;
 	h->delayed_ref_elem.seq = 0;
 	h->type = type;
 	h->allocating_chunk = false;
@@ -402,6 +402,7 @@ again:
 		h->block_rsv = &root->fs_info->trans_block_rsv;
 		h->bytes_reserved = num_bytes;
 	}
+	h->qgroup_reserved = qgroup_reserved;
 
 got_it:
 	btrfs_record_root_in_trans(h, root);