netfilter: ecache: use dedicated list for event redelivery

This disentangles event redelivery and the percpu dying list.

Because entries are now stored on a dedicated list, all
entries are in NFCT_ECACHE_DESTROY_FAIL state and all entries
still have confirmed bit set -- the reference count is at least 1.

The 'struct net' back-pointer can be removed as well.

The pcpu dying list will be removed eventually, it has no functionality.

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