netfilter: nf_tables: add and use helper for module autoload

module autoload is problematic, it requires dropping the mutex that
protects the transaction.  Once the mutex has been dropped, another
client can start a new transaction before we had a chance to abort
current transaction log.

This helper makes sure we first zap the transaction log, then
drop mutex for module autoload.

In case autload is successful, the caller has to reply entire
message anyway.

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