ceph: just call get_session in __ceph_lookup_mds_session

I originally thought there was a potential race here, but the fact
that this is called with the mdsc->mutex held, ensures that the
last reference to the session can't be put here.

Still, it's clearer to just return the value from get_session here,
and may prevent a bug later if we ever rework this code to be less
reliant on mutexes.

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