pptp: lookup route with the proper net namespace

pptp always use init_net as the net namespace to lookup
route, this will cause route lookup failed in container.

because we already set the correct net namespace to struct
sock in pptp_create,so fix this by using sock_net(sk) to
replace &init_net.
Signed-off-by: NGao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ppp/pptp.c

@@ -189,7 +189,7 @@ static int pptp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
 	if (sk_pppox(po)->sk_state & PPPOX_DEAD)
 		goto tx_error;
 
-	rt = ip_route_output_ports(&init_net, &fl4, NULL,
+	rt = ip_route_output_ports(sock_net(sk), &fl4, NULL,
 				   opt->dst_addr.sin_addr.s_addr,
 				   opt->src_addr.sin_addr.s_addr,
 				   0, 0, IPPROTO_GRE,
@@ -468,7 +468,7 @@ static int pptp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	po->chan.private = sk;
 	po->chan.ops = &pptp_chan_ops;
 
-	rt = ip_route_output_ports(&init_net, &fl4, sk,
+	rt = ip_route_output_ports(sock_net(sk), &fl4, sk,
 				   opt->dst_addr.sin_addr.s_addr,
 				   opt->src_addr.sin_addr.s_addr,
 				   0, 0,