netfilter: ecache: Refine the nf_ct_deliver_cached_events

netfilter: ecache: Refine the nf_ct_deliver_cached_events

1. Remove single !events condition check to deliver the missed event
even though there is no new event happened.

Consider this case:
1) nf_ct_deliver_cached_events is invoked at the first time, the
event is failed to deliver, then the missed is set.
2) nf_ct_deliver_cached_events is invoked again, but there is no
any new event happened.
The missed event is lost really.

It would try to send the missed event again after remove this check.
And it is ok if there is no missed event because the latter check
!((events | missed) & e->ctmask) could avoid it.

2. Correct the return value check of notify->fcn.
When send the event successfully, it returns 0, not postive value.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>