ipv6: add knob to send unsolicited ND on link-layer address change

This patch introduces a new knob ndisc_notify. If enabled, the kernel
will transmit an unsolicited neighbour advertisement on link-layer address
change to update the neighbour tables of the corresponding hosts more quickly.

This is the equivalent to arp_notify in ipv4 world.

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>