When looking up an l2tp socket, we must consider a null netdevice id as
wild card. There are currently two problems caused by
__l2tp_ip_bind_lookup() not considering 'dif' as wild card when set to 0:

  * A socket bound to a device (i.e. with sk->sk_bound_dev_if != 0)
    never receives any packet. Since __l2tp_ip_bind_lookup() is called
    with dif == 0 in l2tp_ip_recv(), sk->sk_bound_dev_if is always
    different from 'dif' so the socket doesn't match.

  * Two sockets, one bound to a device but not the other, can be bound
    to the same address. If the first socket binding to the address is
    the one that is also bound to a device, the second socket can bind
    to the same address without __l2tp_ip_bind_lookup() noticing the
    overlap.

To fix this issue, we need to consider that any null device index, be
it 'sk->sk_bound_dev_if' or 'dif', matches with any other value.
We also need to pass the input device index to __l2tp_ip_bind_lookup()
on reception so that sockets bound to a device never receive packets
from other devices.

This patch fixes l2tp_ip6 in the same way.

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