btrfs: qgroup: clear STATUS_FLAG_ON in disabling quota.

we forgot to clear STATUS_FLAG_ON in quota_disable(), it
will cause a problem shown as below:

	# mount /dev/sdc /mnt
	# btrfs quota enable /mnt
	# btrfs quota disable /mnt
	# btrfs quota rescan /mnt
	quota rescan started <--- expecting it fail here.
	# echo $?
	0

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/qgroup.c

@@ -966,6 +966,7 @@ int btrfs_quota_disable(struct btrfs_trans_handle *trans,
 	fs_info->pending_quota_state = 0;
 	quota_root = fs_info->quota_root;
 	fs_info->quota_root = NULL;
+	fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_ON;
 	spin_unlock(&fs_info->qgroup_lock);
 
 	btrfs_free_qgroup_config(fs_info);