generic/774: reduce file size

generic/774: reduce file size

We've gotten complaints about this test taking hours to run and
producing stall warning on test VMs with a large number of cpu cores.  I
think this is due to the maximum atomic write unit being very large on
XFS where we can fall back to a software-based out of place write
implementation.

On the victim machine, the atomic write max is 4MB and there are 24
CPUs.  As a result, aw_bsize to be 1MB, so the file size is
1MB * 24 * 2 * 100 == 4.8GB.  I set up a test machine with fast storage
and 24 CPUs, and the atomic writes poked along at 25MB/s and the total
runtime was 300s.  On spinning rust those stats will be much worse.

Let's try backing the file size off by 10x and see if that eases the
complaints.

Cc: fstests@vger.kernel.org # v2025.10.20
Fixes: 9117fb93b41c38 ("generic: Add atomic write test using fio verify on file mixed mappings")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>