null_blk: Fix return value of nullb_device_power_store()

jira LE-1907
Rebuild_History Non-Buildable kernel-5.14.0-427.40.1.el9_4
commit-author Damien Le Moal <dlemoal@kernel.org>
commit d9ff882

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 <dlemoal@kernel.org>
	Reviewed-by: Yu Kuai <yukuai3@huawei.com>
	Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Link: https://lore.kernel.org/r/20240527043445.235267-1-dlemoal@kernel.org
	Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit d9ff882)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

drivers/block/null_blk/main.c

@@ -455,6 +455,7 @@ 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)) {
 			mutex_lock(&lock);