nbd: clear_sock on netlink disconnect

This is what the ioctl based nbd disconnect does as well.  Without this
the device will just sit there and wait for the connection to go away
(or IO to occur) before the device gets torn down.  Instead clear
everything up on our end so the configuration goes away as quickly as
possible.

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