xfstests: use blkid raw mode

xfstests: use blkid raw mode

blkid without options usually gets no probe results at all just after
creating a filesystem.  This problem is due to the cache that blkid
uses in it's default mode, and is unlikely to get fixed.  Use the -p
option to bypass the cache layer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>