generic/767: only test the hardware atomic write unit

This test sets up scsi_debug so that we can test the fs and block layer
code for hardware-accelerated atomic writes (and not just a software
fallback).  However, the userspace ABI for detecting atomic write
geometry has changed since the start of development (to include said
software fallback) so we must add some extra code to find the real
hardware capabilities, and base the write sizes based on that.

This fixes a test failure with 32k blocksizes because the advertised
atomic_write_unit_max is 128M and fallocate quickly runs out of space.

While we're at it fix a stupid variable usage bug in the loop.

Cc: fstests@vger.kernel.org # v2025.07.13
Fixes: fa8694c823d853 ("generic: various atomic write tests with hardware and scsi_debug")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>