mac802154: Fix potential RCU dereference issue in mac802154_scan_worker

mac802154: Fix potential RCU dereference issue in mac802154_scan_worker

In the `mac802154_scan_worker` function, the `scan_req->type` field was
accessed after the RCU read-side critical section was unlocked. According
to RCU usage rules, this is illegal and can lead to unpredictable
behavior, such as accessing memory that has been updated or causing
use-after-free issues.

This possible bug was identified using a static analysis tool developed
by myself, specifically designed to detect RCU-related issues.

To address this, the `scan_req->type` value is now stored in a local
variable `scan_req_type` while still within the RCU read-side critical
section. The `scan_req_type` is then used after the RCU lock is released,
ensuring that the type value is safely accessed without violating RCU
rules.

Fixes: e2c3e6f53a7a ("mac802154: Handle active scanning")
Cc: stable@vger.kernel.org
Signed-off-by: Jiawei Ye <jiawei.ye@foxmail.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://lore.kernel.org/tencent_3B2F4F2B4DA30FAE2F51A9634A16B3AD4908@qq.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>