netfilter: nf_flow_table: do not flow offload deleted conntrack entries

Conntrack entries can be deleted by the masquerade module. In that case,
flow offload should be deleted too, but GC and data-path of flow offload
do not check for conntrack status bits, hence flow offload entries will
be removed only by the timeout.

Update garbage collector and data-path to check for ct->status. If
IPS_DYING_BIT is set, garbage collector removes flow offload entries and
data-path routine ignores them.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>