ceph: clean up argument lists to __prepare_send_request and __send_request

We can always get the mdsc from the session, so there's no need to pass
it in as a separate argument. Pass the session to __prepare_send_request
as well, to prepare for later patches that will need to access it.

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