cred: crget(): Compute initial 'cr_agroups' with nitems()

No functional change (intended).

Reviewed by: mhorne, emaste
Approved by: markj (mentor)
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46910

(cherry picked from commit f5b46856918cc6cc3ea49e3ab95163d318563ece)