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