cifs: Fix xid leak in cifs_flock()

If not flock, before return -ENOLCK, should free the xid,
otherwise, the xid will be leaked.

Fixes: d0677992

 ("cifs: add support for flock")
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Steve French <stfrench@microsoft.com>