[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>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>