netfilter: ipvs: merge ipv4 + ipv6 icmp reply handlers

Similar to earlier patches: allow ipv4 and ipv6 to use the
same handler.  ipv4 and ipv6 specific actions can be done by
checking state->pf.

v2: split the pf == NFPROTO_IPV4 check (Julian Anastasov)

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>