netfilter: nfnetlink: allow commit to fail

->commit() cannot fail at the moment.

Followup-patch adds kmalloc calls in the commit phase, so we'll need
to be able to handle errors.

Make it so that -EGAIN causes a full replay, and make other errors
cause the transaction to fail.

Failing is ok from a consistency point of view as long as we
perform all actions that could return an error before
we increment the generation counter and the base seq.

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