xfs/520: disable external devices

xfs/520: disable external devices

This is a regression test for a specific bug that requires a specific
configuration of the data device.  Realtime volumes and external logs
don't affect the efficacy of the test, but the test can fail mkfs if the
realtime device is very large.

Therefore, unset USE_EXTERNAL so that we always run this regression
test, even if the tester enabled realtime.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>