netfilter: flowtable: Set offload timeout when adding flow

On heavily loaded systems the GC can take time to go over all existing
conns and reset their timeout. At that time other calls like from
nf_conntrack_in() can call of nf_ct_is_expired() and see the conn as
expired. To fix this when we set the offload bit we should also reset
the timeout instead of counting on GC to finish first iteration over
all conns before the initial timeout.

Fixes: 90964016

 ("netfilter: nf_conntrack: add IPS_OFFLOAD status bit")
Signed-off-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>