netfilter: nf_tables: use correct integer types

Sparse tool complains about mixing of different endianess
types, so use the correct ones.

Add type casts where needed.

objdiff shows no changes except in nft_tunnel (type is changed).

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