netfilter: utils: move nf_ip6_checksum* from ipv6 to utils

similar to previous change, this also allows to remove it
from nf_ipv6_ops and avoid the indirection.

It also removes the bogus dependency of nf_conntrack_ipv6 on ipv6 module:
ipv6 checksum functions are built into kernel even if CONFIG_IPV6=m,
but ipv6/netfilter.o isn't.

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