netfilter: labels: don't emit ct event if labels were not changed

make the replace function only send a ctnetlink event if the contents
of the new set is different.

Otherwise 'ct label set ct label | bar'

will cause netlink event storm since we "replace" labels for each packet.

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