datapath: Check that netdev vport is fully initialized.

datapath: Check that netdev vport is fully initialized.

Starting in 2.6.37 we have our own flag for identifying net_devices
as being attached to OVS.  However, it's possible to receive packets
before this flag has been applied, resulting in a NULL vport when
processing the packet.  This checks to make sure that the vport is
valid instead of crashing.

Bug #5675

Reported-by: Brad Hall <brad@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>