[PPPOE]: Advertise PPPoE MTU

PPPoE must advertise the underlying device's MTU via the ppp channel
descriptor structure, as multilink functionality depends on it.
Signed-off-by: Michal Ostrowski <mostrows@earthlink.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

drivers/net/pppoe.c

@@ -601,6 +601,7 @@ static int pppoe_connect(struct socket *sock, struct sockaddr *uservaddr,
 		po->chan.hdrlen = (sizeof(struct pppoe_hdr) +
 				   dev->hard_header_len);
 
+		po->chan.mtu = dev->mtu - sizeof(struct pppoe_hdr);
 		po->chan.private = sk;
 		po->chan.ops = &pppoe_chan_ops;