netfilter: ecache: add common helper for nf_conntrack_eventmask_report

nf_ct_deliver_cached_events and nf_conntrack_eventmask_report are very
similar.  Split nf_conntrack_eventmask_report into a common helper
function that can be used for both cases.

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