l2tp: fix userspace reception on plain L2TP sockets

As pppol2tp_recv() never queues up packets to plain L2TP sockets,
pppol2tp_recvmsg() never returns data to userspace, thus making
the recv*() system calls unusable.

Instead of dropping packets when the L2TP socket isn't bound to a PPP
channel, this patch adds them to its reception queue.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>