hwt: On attach, ensure owner is a target effective GID's member

This restores a check that existed prior to commit be1f7435ef218b1d
("kern: start tracking cr_gid outside of cr_groups[]").

Fixes: be1f7435ef218b1d ("kern: start tracking cr_gid outside of cr_groups[]")
MFC after: 9 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52253