md: make sure desc_nr less than MD_SB_DISKS

commit 3b7436cc9449d5ff7fa1c1fd5bc3edb6402ff5b8 upstream.

For super_90_load, we need to make sure 'desc_nr' less
than MD_SB_DISKS, avoiding invalid memory access of 'sb->disks'.

Fixes: 228fc7d76db6 ("md: avoid invalid memory access for array sb->dev_roles")
Signed-off-by: NYufen Yu <yuyufen@huawei.com>
Signed-off-by: NSong Liu <songliubraving@fb.com>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Acked-by: NCaspar Zhang <caspar@linux.alibaba.com>

drivers/md/md.c

@@ -1096,6 +1096,7 @@ static int super_90_load(struct md_rdev *rdev, struct md_rdev *refdev, int minor
 	/* not spare disk, or LEVEL_MULTIPATH */
 	if (sb->level == LEVEL_MULTIPATH ||
 		(rdev->desc_nr >= 0 &&
+		 rdev->desc_nr < MD_SB_DISKS &&
 		 sb->disks[rdev->desc_nr].state &
 		 ((1<<MD_DISK_SYNC) | (1 << MD_DISK_ACTIVE))))
 		spare_disk = false;