nbd: multicast dead link notifications

Provide a mechanism to notify userspace that there's been a link problem
on a NBD device.  This will allow userspace to re-establish a connection
and provide the new socket to the device without disrupting the device.

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