netfilter: nf_tables: skip synchronize_rcu if transaction log is empty

After processing the transaction log, the remaining entries of the log
need to be released.

However, in some cases no entries remain, e.g. because the transaction
did not remove anything.

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