netfilter: move checksum_partial indirection to struct nf_ipv6_ops

We cannot make a direct call to nf_ip6_checksum_partial() because that
would result in autoloading the 'ipv6' module because of symbol
dependencies.  Therefore, define checksum_partial indirection in
nf_ipv6_ops where this really belongs to.

For IPv4, we can indeed make a direct function call, which is faster,
given IPv4 is built-in in the networking code by default. Still,
CONFIG_INET=n and CONFIG_NETFILTER=y is possible, so define empty inline
stub for IPv4 in such case.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>