netfilter: nf_tables_offload: Check for the NETDEV_UNREGISTER event

Check for the NETDEV_UNREGISTER event from the nft_offload_netdev_event
function, which is the event that actually triggers the clean up.

Fixes: 06d392cb

 ("netfilter: nf_tables_offload: remove rules when the device unregisters")
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>