vport-internal: Set vport to NULL when detaching.

vport-internal: Set vport to NULL when detaching.

'struct net_device' is refcounted and can stick around for quite a
while if someone is still holding a reference to it.  However, we
free the vport that it is attached to in the next RCU grace period
after detach.  This assigns the vport to NULL on detach and adds
appropriate checks.