scsi: hisi_sas: Limit users changing debugfs BIST count value

[Fred Kimmy]

[Bug Description]
After the cnt error code is set to 100, no error is reported.

[Steps to Reproduce]
1)dmesg -C
2)lsblk
3)lsscsi -p
4)dmesg
5)lsmod | grep hisi_sas_v3
6)mount | grep debugfs
7)rmmod hisi_sas_v3_hw
8)rmmod hisi_sas_main
9)modprobe hisi_sas_main debugfs_enable=1
10)modprobe hisi_sas_v3_hw
11)cd /sys/kernel/debug/hisi_sas/0000\:74\:02.0/
12)ll
13)cd bist/
14)cat enable
15)cat cnt
16)echo 49 > cnt
17)echo 100 > cnt
18)dmesg

[Actual Results]
echo ok

[Expected Results]
echo fail

[Reproducibility]
Conditionally recurs.

[Additional information]
 Ubuntu-hwe-5.13-5.13.0-35.40_20.04.1

[Resolution]
scsi: hisi_sas: Limit users changing debugfs BIST count value
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.19-rc6&id=286ce4c65fbdf5eb9d4d5f4e4997c4e32bf1b073

[Ike Panhc (ikepanhc)]

Looks good to me. I will try to backport and reproduce.

[Ike Panhc (ikepanhc)]

Patch has been merged into Ubuntu focal HWE kernel via upstream v5.15.34.