ipv6: flush nd cache on IFF_NOARP change

This patch is the IPv6 equivalent of commit
6c8b4e3f ("arp: flush arp cache on IFF_NOARP change")

Without it, we keep buggy neighbours in the cache, with destination
MAC address equal to our own MAC address.

Tested:
 tcpdump -i eth0 -s 0 ip6 -n -e &
 ip link set dev eth0 arp off
 ping6 remote   // sends buggy frames
 ip link set dev eth0 arp on
 ping6 remote   // should work once kernel is patched

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Mario Fanelli <mariofanelli@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>