Bluetooth: Fix decrementing reference count twice in releasing socket

When releasing socket, it is possible to enter hci_sock_release() and
hci_sock_dev_event(HCI_DEV_UNREG) at the same time in different thread.
The reference count of hdev should be decremented only once from one of
them but if storing hdev to local variable in hci_sock_release() before
detached from socket and setting to NULL in hci_sock_dev_event(),
hci_dev_put(hdev) is unexpectedly called twice. This is resolved by
referencing hdev from socket after bt_sock_unlink() in
hci_sock_release().

Reported-by: syzbot+fdc00003f4efff43bc5b@syzkaller.appspotmail.com
Signed-off-by: Myungho Jung <mhjungk@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/hci_sock.c

@@ -831,8 +831,6 @@ static int hci_sock_release(struct socket *sock)
 	if (!sk)
 		return 0;
 
-	hdev = hci_pi(sk)->hdev;
-
 	switch (hci_pi(sk)->channel) {
 	case HCI_CHANNEL_MONITOR:
 		atomic_dec(&monitor_promisc);
@@ -854,6 +852,7 @@ static int hci_sock_release(struct socket *sock)
 
 	bt_sock_unlink(&hci_sk_list, sk);
 
+	hdev = hci_pi(sk)->hdev;
 	if (hdev) {
 		if (hci_pi(sk)->channel == HCI_CHANNEL_USER) {
 			/* When releasing a user channel exclusive access,