proc: prevent changes to overridden credentials

Prevent userspace from changing the the /proc/PID/attr values if the
task's credentials are currently overriden.  This not only makes sense
conceptually, it also prevents some really bizarre error cases caused
when trying to commit credentials to a task with overridden
credentials.

Cc: <stable@vger.kernel.org>
Reported-by: "chengjian (D)" <cj.chengjian@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Acked-by: John Johansen <john.johansen@canonical.com>
Acked-by: James Morris <james.morris@microsoft.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>