pppoe: fix reference counting in PPPoE proxy

[ Upstream commit 29e73269aa4d36f92b35610c25f8b01c789b0dc8 ]

Drop reference on the relay_po socket when __pppoe_xmit() succeeds.
This is already handled correctly in the error path.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>