netfilter: nft_flow_offload: wait for garbage collector to run after cleanup

If netdevice goes down, then flowtable entries are scheduled to be
removed. Wait for garbage collector to have a chance to run so it can
delete them from the hashtable.

The flush call might sleep, so hold the nfnl mutex from
nft_flow_table_iterate() instead of rcu read side lock. The use of the
nfnl mutex is also implicitly fixing races between updates via nfnetlink
and netdevice event.

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