groups: Consolidate the setgroups permission checks

Today there are 3 instances of setgroups and due to an oversight their
permission checking has diverged.  Add a common function so that
they may all share the same permission checking code.

This corrects the current oversight in the current permission checks
and adds a helper to avoid this in the future.

A user namespace security fix will update this new helper, shortly.

Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --cc arch/s390/kernel/compat_linux.c
Simple merge

diff --cc include/linux/cred.h
Simple merge

diff --cc kernel/groups.c
Simple merge

diff --cc kernel/uid16.c
Simple merge