netfilter: nfnetlink: add missing __be16 cast

Sparse flags this as suspicious, because this compares
integer with a be16 with no conversion.

Its a compat check for old userspace that sends host byte order,
so force a be16 cast here.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>