nbd: Consistently use request pointer in debug messages.

Existing dev_dbg messages sometimes identify request using request
pointer, sometimes using nbd_cmd pointer. This makes it hard to
follow request flow. Consistently use request pointer instead.

Reviewed-by: Josef Bacik <jbacik@toxicpanda.com>
Signed-off-by: Kevin Vigor <kvigor@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>