netfilter: ctnetlink: send event when conntrack label was modified

netfilter: ctnetlink: send event when conntrack label was modified

commit 0ceabd83875b72a29f33db4ab703d6ba40ea4c58
(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 9b21f6a90924dfe8e5e686c314ddb441fb06501e
(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>