netfilter: conntrack: merge ipv4+ipv6 confirm functions

No need to have distinct functions.  After merge, ipv6 can avoid
protooff computation if the connection neither needs sequence adjustment
nor helper invocation -- this is the normal case.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>