ceph: remove incorrect session state check

Once the session is opened the s->s_ttl will be set, and when receiving
a new mdsmap and the MDS map is changed, it will be possibly will close
some sessions and open new ones. And then some sessions will be in
CLOSING state evening without unmounting.

URL: https://tracker.ceph.com/issues/54979
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>