ceph: drop session argument to ceph_fill_trace

Just get it from r_session since that's what's always passed in.

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