ipfilter: Avoid negative array indicies

Array indices must always be posive. We avoid this by making each index
unsigned. This mitigates out-of-bounds reads and writes.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: glebius
MFC after: 3 days
Differential revision: https://reviews.freebsd.org/D55260