md/raid5: fix possible null-pointer dereferences in raid5_store_group_thread_cnt()

In the Linux kernel, the following vulnerability has been resolved:

md/raid5: fix possible null-pointer dereferences in raid5_store_group_thread_cnt()

The variable mddev->private is first assigned to conf and then checked:

conf = mddev->private;

if (!conf) ...

If conf is NULL, then mddev->private is also NULL. In this case,

null-pointer dereferences can occur when calling raid5_quiesce():

raid5_quiesce(mddev, true);

raid5_quiesce(mddev, false);

since mddev->private is assigned to conf again in raid5_quiesce(), and conf

is dereferenced in several places, for example:

conf->quiesce = 0;

wake_up(&conf->wait_for_quiescent);

To fix this issue, the function should unlock mddev and return before

invoking raid5_quiesce() when conf is NULL, following the existing pattern

in raid5_change_consistency_policy().