netfilter: conntrack: move ecache dwork to net_generic infra

dwork struct is large (>128 byte) and not needed when conntrack module
is not loaded.

Place it in net_generic data instead.  The struct net dwork member is now
obsolete and will be removed in a followup patch.

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