netfilter: nf_tables: do not hold reference on netdevice from preparation phase

The netfilter netdevice event handler hold the nfnl_lock mutex, this
avoids races with a device going away while such device is being
attached to hooks from the netlink control plane. Therefore, either
control plane bails out with ENOENT or netdevice event path waits until
the hook that is attached to net_device is registered.

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