[PPPOE]: Key connections properly on local device.

It is based on the assumption that an interface's ifindex is basically
an alias for a local MAC address, so incoming packets now are matched
to sockets based on remote MAC, session id, and ifindex of the
interface the packet came in on/the socket was bound to by connect().

For relayed packets, the socket that's used for relaying is selected
based on destination MAC, session ID and the interface index of the
interface whose name currently matches the name requested by userspace
as the relaying source interface.

Signed-off-by: David S. Miller <davem@davemloft.net>