nbd: allow multiple disconnects to be sent

There's no reason to limit ourselves to one disconnect message per
socket.  Sometimes networks do strange things, might as well let
sysadmins hit the panic button as much as they want.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>