wifi: cfg80211: fix RCU dereference in __cfg80211_bss_update

Replace rcu_dereference() with rcu_access_pointer() since we hold
the lock here (and aren't in an RCU critical section).

Fixes: 32af9a9e ("wifi: cfg80211: free beacon_ies when overridden from hidden BSS")
Reported-and-tested-by: syzbot+864a269c27ee06b58374@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Link: https://msgid.link/tencent_BF8F0DF0258C8DBF124CDDE4DD8D992DCF07@qq.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/scan.c

@@ -1864,7 +1864,7 @@ __cfg80211_bss_update(struct cfg80211_registered_device *rdev,
 					 &hidden->hidden_list);
 				hidden->refcount++;
 
-				ies = (void *)rcu_dereference(new->pub.beacon_ies);
+				ies = (void *)rcu_access_pointer(new->pub.beacon_ies);
 				rcu_assign_pointer(new->pub.beacon_ies,
 						   hidden->pub.beacon_ies);
 				if (ies)