linux: Simplify further getgroups() after 'cr_gid' not in cr_groups[]

No functional change (intended).

While here, fix/improve style a bit and in setgroups().

MFC after: 5 days
MFC to: stable/15
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52276

(cherry picked from commit a207833f4fed5431cac853c082fa34dc8f33cba6)