netfilter: flowtable: remove dying bit, use teardown bit instead

netfilter: flowtable: remove dying bit, use teardown bit instead

The dying bit removes the conntrack entry if the netdev that owns this
flow is going down. Instead, use the teardown mechanism to push back the
flow to conntrack to let the classic software path decide what to do
with it.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>