ceph: fix potential use-after-free

__unregister_session() free the session if it drops the last
reference. We should grab an extra reference if we want to use
session after __unregister_session().

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