netfilter: nf_conntrack_netbios_ns: fix helper module alias

The helper gets registered as 'netbios-ns', not netbios_ns.
Intentionally not adding a fixes-tag because i don't want this to go to
stable. This wasn't noticed for a very long time so no so no need to risk
regressions.

Reported-by: Yi Chen <yiche@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>