ceph: don't WARN if we're force umounting

Force umount will try to close the sessions by setting the session
state to _CLOSING. We don't want to WARN in this situation, since it's
expected.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>