netfilter: flowtable: refresh flow if hardware offload fails

If nf_flow_offload_add() fails to add the flow to hardware, then the
NF_FLOW_HW_REFRESH flag bit is set and the flow remains in the flowtable
software path.

If flowtable hardware offload is enabled, this patch enqueues a new
request to offload this flow to hardware.

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