xfstests: ensure selinux xattrs don't interfere with quota tests

xfstests: ensure selinux xattrs don't interfere with quota tests

219 and 235 were failing on ext3 with selinux, because the extra
xattrs upped the quota usage.

Do the same trick we've used in the past to mount with an selinux
global context when doing quota tests.

Note the gross hack for remount,ro, though.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>