ipvlan: add L2 check for packets arriving via virtual devices

Packets that don't have dest mac as the mac of the master device should
not be entertained by the IPvlan rx-handler. This is mostly true as the
packet path mostly takes care of that, except when the master device is
a virtual device. As demonstrated in the following case -

  ip netns add ns1
  ip link add ve1 type veth peer name ve2
  ip link add link ve2 name iv1 type ipvlan mode l2
  ip link set dev iv1 netns ns1
  ip link set ve1 up
  ip link set ve2 up
  ip -n ns1 link set iv1 up
  ip addr add 192.168.10.1/24 dev ve1
  ip -n ns1 addr 192.168.10.2/24 dev iv1
  ping -c2 192.168.10.2
  <Works!>
  ip neigh show dev ve1
  ip neigh show 192.168.10.2 lladdr <random> dev ve1
  ping -c2 192.168.10.2
  <Still works! Wrong!!>

This patch adds that missing check in the IPvlan rx-handler.

Reported-by: Amit Sikka <amit.sikka@ericsson.com>
Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>