netfilter: nf_tables: cache device name in flowtable object

Devices going away have to grab the nfnl_lock from the netdev event path
to avoid races with control plane updates.

However, netlink dumps in netfilter do not hold nfnl_lock mutex. Cache
the device name into the objects to avoid an use-after-free situation
for a device that is going away.

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