netfilter: nf_tables: return immediately on empty commit

When running 'nft flush ruleset' while no rules exist, we will increment
the generation counter and announce a new genid to userspace, yet
nothing had changed in the first place.

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