netfilter: nf_tables: Pass struct net in nft_pktinfo

nft_pktinfo is passed on the stack so this does not bloat any in core
data structures.

By centrally computing this information this makes maintence of the code
simpler, and understading of the code easier.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>