netfilter: flowtable: Make sure GC works periodically in idle system

Currently flowtable's GC work is initialized as deferrable, which
means GC cannot work on time when system is idle. So the hardware
offloaded flow may be deleted for timeout, since its used time is
not timely updated.

Resolve it by initializing the GC work as delayed work instead of
deferrable.

Fixes: c29f74e0

 ("netfilter: nf_flow_table: hardware offload support")
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>