LinuxKPI: add struct_group_tagged()

Restructure struct_group() into a wrapper around __struct_group().
Various callers have arguments annotated as /* no foo */ so try to
add all we can find.

Sponsored by: The FreeBSD Foundation
Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D50157

(cherry picked from commit 54d0e1772fb6b83d72725764e0a5d3ead8337673)