MAC/bsdextended: Restore matching subjects' effective GID
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/D52270
(cherry picked from commit ab7186a486347216fde099498471cf847828565c)