netfilter: rpfilter: Remove unused variable 'ret'.

Commit 91a17825

 ("netfilter: rpfilter: Convert
rpfilter_lookup_reverse to new dev helper") removed the need for the
'ret' variable. This went unnoticed because of the __maybe_unused
annotation.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>