pptp: support sockets bound to an interface

use sk_bound_dev_if for route lookup as already done
in most of the other ip_route_output_ports() calls.

Since most PPPoA providers use 10.0.0.138 as default gateway IP
this will allow connections to multiple PPTP providers with the
same IP address over different interfaces.
Signed-off-by: Ulrich Weber <ulrich.weber@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp/pptp.c

@@ -155,7 +155,7 @@ static int pptp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
 				   opt->dst_addr.sin_addr.s_addr,
 				   opt->src_addr.sin_addr.s_addr,
 				   0, 0, IPPROTO_GRE,
-				   RT_TOS(0), 0);
+				   RT_TOS(0), sk->sk_bound_dev_if);
 	if (IS_ERR(rt))
 		goto tx_error;
 
@@ -444,7 +444,8 @@ static int pptp_connect(struct socket *sock, struct sockaddr *uservaddr,
 				   opt->dst_addr.sin_addr.s_addr,
 				   opt->src_addr.sin_addr.s_addr,
 				   0, 0,
-				   IPPROTO_GRE, RT_CONN_FLAGS(sk), 0);
+				   IPPROTO_GRE, RT_CONN_FLAGS(sk),
+				   sk->sk_bound_dev_if);
 	if (IS_ERR(rt)) {
 		error = -EHOSTUNREACH;
 		goto end;