null_blk: Fix return value of nullb_device_power_store()

When powering on a null_blk device that is not already on, the return
value ret that is initialized to be count is reused to check the return
value of null_add_dev(), leading to nullb_device_power_store() to return
null_add_dev() return value (0 on success) instead of "count".
So make sure to set ret to be equal to count when there are no errors.

Fixes: a2db328 ("null_blk: fix null-ptr-dereference while configuring 'power' and 'submit_queues'")
Signed-off-by: Damien Le Moal <[email protected]>
Reviewed-by: Yu Kuai <[email protected]>
Reviewed-by: Kanchan Joshi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

drivers/block/null_blk/main.c

@@ -494,6 +494,7 @@ static ssize_t nullb_device_power_store(struct config_item *item,
 
  set_bit(NULLB_DEV_FL_CONFIGURED, &dev->flags);
  dev->power = newp;
+ ret = count;
  } else if (dev->power && !newp) {
  if (test_and_clear_bit(NULLB_DEV_FL_UP, &dev->flags)) {
  dev->power = newp;