cred: group_is_supplementary(): Use bsearch()

This makes that function use a more efficient version of binary search
instead, and removes one more hand-rolled binary search code from the
tree (and the kernel binary).

Reviewed by: mhorne, emaste
Approved by: markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46907