netfilter: guarantee 8 byte minalign for template addresses

The next change will merge skb->nfct pointer and skb->nfctinfo
status bits into single skb->_nfct (unsigned long) area.

For this to work nf_conn addresses must always be aligned at least on
an 8 byte boundary since we will need the lower 3bits to store nfctinfo.

Conntrack templates are allocated via kmalloc.
kbuild test robot reported
BUILD_BUG_ON failed: NFCT_INFOMASK >= ARCH_KMALLOC_MINALIGN
on v1 of this patchset, so not all platforms meet this requirement.

Do manual alignment if needed,  the alignment offset is stored in the
nf_conn entry protocol area. This works because templates are not
handed off to L4 protocol trackers.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>