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: [e-mail]
Signed-off-by: Jiawei Ye <[e-mail]>
Acked-by: Miquel Raynal <[e-mail]>
Reviewed-by: Przemek Kitszel <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Stefan Schmidt <[e-mail]>

net/mac802154/scan.c

···
 	struct ieee802154_local *local =
 		container_of(work, struct ieee802154_local, scan_work.work);
 	struct cfg802154_scan_request *scan_req;
+	enum nl802154_scan_types scan_req_type;
 	struct ieee802154_sub_if_data *sdata;
 	unsigned int scan_duration = 0;
 	struct wpan_phy *wpan_phy;
···
 	}
 
 	wpan_phy = scan_req->wpan_phy;
+	scan_req_type = scan_req->type;
 	scan_req_duration = scan_req->duration;
 
 	/* Look for the next valid chan */
···
 		goto end_scan;
 	}
 
-	if (scan_req->type == NL802154_SCAN_ACTIVE) {
+	if (scan_req_type == NL802154_SCAN_ACTIVE) {
 		ret = mac802154_transmit_beacon_req(local, sdata);
 		if (ret)
 			dev_err(&sdata->dev->dev,