Simplify dynamic ipfilter sysctls.

Pass the structure offset in arg2 instead of arg1. This avoids
having to undo the pointer arithmetic on arg1. Instead arg2 can
be used directly as an offset relative to the desired structure.

Reviewed by: cy
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D27961

(cherry picked from commit d86d3194955c3063baeed27de4bc84dfe7a7187d)