dmflakey: override SCRATCH_DEV in _init_flakey

dmflakey: override SCRATCH_DEV in _init_flakey

_init_flakey already overrides SCRATCH_LOGDEV and SCRATCH_RTDEV so that
the XFS-specific helpers work fine with external devices.  Do the same
for SCRATCH_DEV itself, so that _scratch_mount and _scratch_unmount just
work, and so that _check_scratch_fs does not need to override the main
device.

This requires some small adjustments in how generic/741 checks that
mounting the underlying device fails, but the new version actually is
simpler than the old one, and in xfs/438 where we need to be careful
where to create the custom dm table.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Anand Jain <asj@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>