stop overriding SCRATCH_{,LOG,RT}DEV v2

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series cleans up various tests to get out of the business of
overriding the scratch devices setup.  This is not only generally a
little ugly, but breaks when mkfs options are incompatible with the
synthesized setups.

My prime example right now are two zoned XFS options:  -r rtreserved,
which only works for zoned RT devices, and breaks as soon as we're
running on non-RT or non-Zoned setups, and -r rtstart which only works
for internal RT zoned RT devices, and breaks as soon as an actual
SCRATCH_RTDEV was used.  There's probably more that we've been
papering over with the try_ options and by scratch_mkfs dropping
options when they conflict.  I plan to remove the need for the latter
in a follow-on series as it leads to hard to debug bugs.

Changes since v1:
 - pass a device to e2fsck in ext4/006
 - add a _require_scratch_size to xfs/521
 - ensure the file system is still mounted when checking it in xfs/528