dtrace: Fix effective GID shown by 'curpsinfo'

Reviewed by: emaste
Fixes: be1f7435ef218b1d ("kern: start tracking cr_gid outside of cr_groups[]")
MFC after: 5 days
MFC to: stable/15
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52274

(cherry picked from commit e4cc7f9930573daeb7f1056d008e424aa4c3fd33)