scsi: scsi_debug: Reduce DEF_ATOMIC_WR_MAX_LENGTH

scsi: scsi_debug: Reduce DEF_ATOMIC_WR_MAX_LENGTH

The default atomic write max length in DEF_ATOMIC_WR_MAX_LENGTH is
excessively large.

For 512B LBS, we would get a 4MB max, but due to block layer atomic write
restrictions this is limited to 512KB.

Reduce DEF_ATOMIC_WR_MAX_LENGTH to a value which would be more realistic
(for a real device supporting atomic writes), 64KB.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20250501100241.930071-1-john.g.garry@oracle.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>