generic: don't put loop control files on the scratch mount

generic: don't put loop control files on the scratch mount

If we're doing write/overwrite/snapshot/resource exhaustion tests on
the scratch device, use the test directory to hold the loop
termination signal files.  This way we don't run infinitely because
we can't create the flag due to ENOSPC.

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