Bluetooth: Check if mode is supported on getsockopt

Add this check to getsockopt makes possible to fail early instead of
waiting until listen / connect.

Signed-off-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Acked-by: Gustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/l2cap.c

@@ -1814,9 +1814,22 @@ static int l2cap_sock_setsockopt_old(struct socket *sock, int optname, char __us
 			break;
 		}
 
+		l2cap_pi(sk)->mode = opts.mode;
+		switch (l2cap_pi(sk)->mode) {
+		case L2CAP_MODE_BASIC:
+			break;
+		case L2CAP_MODE_ERTM:
+		case L2CAP_MODE_STREAMING:
+			if (enable_ertm)
+				break;
+			/* fall through */
+		default:
+			err = -EINVAL;
+			break;
+		}
+
 		l2cap_pi(sk)->imtu = opts.imtu;
 		l2cap_pi(sk)->omtu = opts.omtu;
-		l2cap_pi(sk)->mode = opts.mode;
 		l2cap_pi(sk)->fcs  = opts.fcs;
 		l2cap_pi(sk)->max_tx = opts.max_tx;
 		l2cap_pi(sk)->tx_win = (__u8)opts.txwin_size;