netfilter: nf_tables: reduce nft_pktinfo by 8 bytes

structure is reduced from 32 to 24 bytes.  While at it, also check
that iphdrlen is sane, this is guaranteed for NFPROTO_IPV4 but not
for ingress or bridge, so add checks for this.

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