userns: Fix build of drivers/staging/dgrp

Explicitly test for GLOBAL_ROOT_UID and GLOBAL_ROOT_GID
instead of using 0, allowing dgrp to compile and work
properly when user namespace support is enabled.

Cc: Bill Pemberton <wfp5p@virginia.edu>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>