Bluetooth: move adv_instance_cnt read within the device lock

The field adv_instance_cnt is always accessed within a device lock,
except in the function add_advertising. A concurrent remove of an
advertisement with adding another one could result in the if check
"if a new instance was actually added" to not trigger, resulting
in not triggering the "advertising added event".
Signed-off-by: NNiels Dossche <niels.dossche@ugent.be>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/bluetooth/mgmt.c

@@ -8079,7 +8079,7 @@ static int add_advertising(struct sock *sk, struct hci_dev *hdev,
 	u32 flags;
 	u8 status;
 	u16 timeout, duration;
-	unsigned int prev_instance_cnt = hdev->adv_instance_cnt;
+	unsigned int prev_instance_cnt;
 	u8 schedule_instance = 0;
 	struct adv_info *next_instance;
 	int err;
@@ -8130,6 +8130,8 @@ static int add_advertising(struct sock *sk, struct hci_dev *hdev,
 		goto unlock;
 	}
 
+	prev_instance_cnt = hdev->adv_instance_cnt;
+
 	err = hci_add_adv_instance(hdev, cp->instance, flags,
 				   cp->adv_data_len, cp->data,
 				   cp->scan_rsp_len,