scsi: hisi_sas: Add latest_dump for the debugfs dump

scsi: hisi_sas: Add latest_dump for the debugfs dump

Before that, after the user triggers the dump, the latest dump
information can be viewed in the directory with the maximum number in
the dump directory.

After this series patch, the driver creates all debugfs directories and
files during initialization. Therefore, users cannot know the directory
where the latest dump information is stored. So, add latest_dump file to
notify users where the latest dump information is stored.

Signed-off-by: Yihang Li <liyihang9@huawei.com>
Link: https://lore.kernel.org/r/20241008021822.2617339-14-liyihang9@huawei.com
Reviewed-by: Xingui Yang <yangxingui@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>