ipv6: Send unsolicited neighbour advertismements when notified

The NETDEV_NOTIFY_PEERS notifier is a request to send such
advertisements following migration to a different physical link,
e.g. virtual machine migration.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>