netfilter: Add the missed return value check of register_netdevice_notifier

There are some codes of netfilter module which did not check the return
value of register_netdevice_notifier. Add the checks now.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>