netfilter: ctnetlink: Support offloaded conntrack entry deletion

Currently, offloaded conntrack entries (flows) can only be deleted
after they are removed from offload, which is either by timeout,
tcp state change or tc ct rule deletion. This can cause issues for
users wishing to manually delete or flush existing entries.

Support deletion of offloaded conntrack entries.

Example usage:
 # Delete all offloaded (and non offloaded) conntrack entries
 # whose source address is 1.2.3.4
 $ conntrack -D -s 1.2.3.4
 # Delete all entries
 $ conntrack -F

Signed-off-by: Paul Blakey <paulb@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>