netfilter: nf_tables: use dedicated mutex to guard transactions

Continue to use nftnl subsys mutex to protect (un)registration of hook types,
expressions and so on, but force batch operations to do their own
locking.

This allows distinct net namespaces to perform transactions in parallel.

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