netfilter: nat: don't register device notifier twice

netfilter: nat: don't register device notifier twice

Otherwise, we get notifier list corruption.

This is the most simple fix: remove the device notifier call chain
from the ipv6 masquerade register function and handle it only
in the ipv4 version.

The better fix is merge
nf_nat_masquerade_ipv4/6_(un)register_notifier
  into a single
nf_nat_masquerade_(un)register_notifiers

but to do this its needed to first merge the two masquerade modules
into a single xt_MASQUERADE.

Furthermore, we need to use different refcounts for ipv4/ipv6
until we can merge MASQUERADE.

Fixes: d1aca8ab3104a ("netfilter: nat: merge ipv4 and ipv6 masquerade functionality")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>