ceph: remove req from unsafe list when unregistering it

There's no reason a request should ever be on a s_unsafe list but not
in the request tree.

Cc: stable@vger.kernel.org
Link: http://tracker.ceph.com/issues/18474


Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Yan, Zheng <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>