arp: flush arp cache on IFF_NOARP change

IFF_NOARP affects what kind of neighbor entries are created
(nud NOARP or nud INCOMPLETE). If the flag changes, flush the arp
cache to refresh all entries.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
v2->v3: shortened notifier_info struct name
Signed-off-by: David S. Miller <davem@davemloft.net>