Merge branch 'icmp6-support-rfc-4884'

Merge branch 'icmp6-support-rfc-4884'

Willem de Bruijn says:

====================
icmp6: support rfc 4884

Extend the feature merged earlier this week for IPv4 to IPv6.

I expected this to be a single patch, but patch 1 seemed better to be
stand-alone

patch 1: small fix in length calculation
patch 2: factor out ipv4-specific
patch 3: add ipv6

changes v1->v2: add missing static keyword in patch 3
====================

Signed-off-by: David S. Miller <davem@davemloft.net>