Bluetooth: Disallow to change L2CAP_OPTIONS values when connected

L2CAP doesn't permit change like MTU, FCS, TxWindow values while the
connection is alive, we can only set that before the
connection/configuration process. That can lead to bugs in the L2CAP
operation.
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>

net/bluetooth/l2cap.c

@@ -1950,6 +1950,11 @@ static int l2cap_sock_setsockopt_old(struct socket *sock, int optname, char __us
 
 	switch (optname) {
 	case L2CAP_OPTIONS:
+		if (sk->sk_state == BT_CONNECTED) {
+			err = -EINVAL;
+			break;
+		}
+
 		opts.imtu     = l2cap_pi(sk)->imtu;
 		opts.omtu     = l2cap_pi(sk)->omtu;
 		opts.flush_to = l2cap_pi(sk)->flush_to;