netfilter: nf_tables: avoid global info storage

This works because all accesses are currently serialized by nfnl
nf_tables subsys mutex.

If we want to have per-netns locking, we need to make this scratch
area pernetns or allocate it on demand.

This does the latter, its ~28kbyte but we can fallback to vmalloc
so it should be fine.

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