pf: Set the pfik_group for userspace

Userspace relies on this pointer to work out if the kif is a group or
not. It can't use it for anything else, because it's a pointer to a
kernel address. Substitute 0xfeedc0de for 'true', so that we don't leak
kernel memory addresses to userspace.

PR: 255852
Reviewed by: donner
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30284