nbd: stop leaking sockets

This was introduced in the multi-connection patch, we've been leaking
socket's ever since.

Fixes: 9561a7ad ("nbd: add multi-connection support")
cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>