netfilter: ctnetlink: send event when conntrack label was modified

commit 0ceabd83
(netfilter: ctnetlink: deliver labels to userspace) sets the event bit
when we raced with another packet, instead of raising the event bit
when the label bit is set for the first time.

commit 9b21f6a9
(netfilter: ctnetlink: allow userspace to modify labels) forgot to update
the event mask in the "conntrack already exists" case.

Both issues result in CTA_LABELS attribute not getting included in the
conntrack event.

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