xfs/818: fix some design issues

xfs/818: fix some design issues

While QA'ing zoned filesystem support, I discovered some design errors
in this test:

1) Since we're test formatting a sparse file on an xfs filesystem,
there's no need to play games with optimal device size; we can create
a totally sparse file that's the same size as SCRATCH_DEV.

2) mkfs.xfs cannot create realtime files, so if it fails with that,
there's no need to continue the test.

3) If mkfs -p fails for none of the proscribed reasons, it should exit
the test.  The final cat $tmp.mkfs will take care of tweaking the golden
output to register the test failure for further investigation.

Cc: fstests@vger.kernel.org # v2025.03.09
Fixes: 6d39dc34e61e11 ("xfs: test filesystem creation with xfs_protofile")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>