ipfilter: Cast uintmax_t values to size_t when adding to a pointer.

GCC warns about the mismatched sizes on 32-bit platforms where
uintmax_t is larger in size than a pointer.

Reviewed by: imp, cy
Differential Revision: https://reviews.freebsd.org/D36753

(cherry picked from commit 9c4f7e82348b656aef9660f859b76d6e894590aa)