IPVS: Add handling of incoming ICMPV6 messages

Add handling of incoming ICMPv6 messages.
This follows the handling of IPv4 ICMP messages.

Amongst ther things this problem allows IPVS to behave sensibly
when an ICMPV6_PKT_TOOBIG message is received:

This message is received when a realserver sends a packet >PMTU to the
client. The hop on this path with insufficient MTU will generate an
ICMPv6 Packet Too Big message back to the VIP. The LVS server receives
this message, but the call to the function handling this has been
missing. Thus, IPVS fails to forward the message to the real server,
which then does not adjust the path MTU. This patch adds the missing
call to ip_vs_in_icmp_v6() in ip_vs_in() to handle this situation.

Thanks to Rob Gallagher from HEAnet for reporting this issue and for
testing this patch in production (with direct routing mode).

[horms@verge.net.au: tweaked changelog]
Signed-off-by: Julius Volz <julius.volz@gmail.com>
Tested-by: Rob Gallagher <robert.gallagher@heanet.ie>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Patrick McHardy <kaber@trash.net>