xfstests: mount xfs with a context when selinux is on

xfstests: mount xfs with a context when selinux is on

When selinux is on, we get tons of new xattrs, which messes
up all kinds of output.

The simplest way out of this, for now, seems to be to just mount
with a global context instead and skip writing the extra xattrs.

I've been using this internally on Fedora and RHEL for a while now.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Dave Chinner <david@fromorbit.com>