netfilter: ecache: prepare for event notifier merge

This prepares for merge for ct and exp notifier structs.

The 'fcn' member is renamed to something unique.
Second, the register/unregister api is simplified.  There is only
one implementation so there is no need to do any error checking.

Replace the EBUSY logic with WARN_ON_ONCE.  This allows to remove
error unwinding.

The exp notifier register/unregister function is removed in
a followup patch.

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