[NETFILTER]: introduce and use aligned_u64 data type

As proposed by Andi Kleen, this is required esp. for x86_64 architecture,
where 64bit code needs 8byte aligned 64bit data types, but 32bit userspace
apps will only align to 4bytes.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>