hwmon: (pmbus) Fix type confusion in notification logic

hwmon: (pmbus) Fix type confusion in notification logic

Sashiko reports:

At the start of the loop in pmbus_notify(), the code unconditionally casts
every attribute to a struct sensor_device_attribute:

drivers/hwmon/pmbus/pmbus_core.c:pmbus_notify() {
    for (i = 0; i < data->num_attributes; i++) {
        struct device_attribute *da = to_dev_attr(data->group.attrs[i]);
        struct sensor_device_attribute *attr = to_sensor_dev_attr(da);
        int index = attr->index;
...
}

However, data->group.attrs can contain other types like struct
pmbus_samples_reg or struct pmbus_sensor, which only embed a base
struct device_attribute.

If da is a struct pmbus_samples_reg, dev_attr is the last member. Casting
it to struct sensor_device_attribute and reading the index field appears
to access memory past the end of the allocation, which might trigger a
slab-out-of-bounds read.

Additionally, if da is a struct pmbus_sensor, casting it causes the index
field to overlap with the page, phase, and reg fields. Could this produce
a garbage mask on little-endian systems that spuriously matches the target
reg, page, and flags during an alert?

Fix the problem by using struct sensor_device_attr in struct pmbus_sensor
and struct pmbus_label. Since those attributes never trigger a
notification, set the value of attr->index to -1 for them. Use this value
to distinguish from boolean attributes which _can_ trigger a notification
and use the index field to encode mask, page, and register values.

Fixes: f469bde9afd1 ("hwmon: (pmbus/core) Notify hwmon events")
Cc: Vincent Jardin <vjardin@free.fr>
Tested-by: Vincent Jardin <vjardin@free.fr>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>