netfilter: ctnetlink: fix regression in expectation handling

This patch fixes a regression (introduced by myself in commit 19abb7b0:
netfilter: ctnetlink: deliver events for conntracks changed from
userspace) that results in an expectation re-insertion since
__nf_ct_expect_check() may return 0 for expectation timer refreshing.

This patch also removes a unnecessary refcount bump that
pretended to avoid a possible race condition with event delivery
and expectation timers (as said, not needed since we hold a
reference to the object since until we finish the expectation
setup). This also merges nf_ct_expect_related_report() and
nf_ct_expect_related() which look basically the same.

Reported-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>