vlan: Fix vlan-in-vlan crashes.

As analyzed by Patrick McHardy, vlan needs to reset it's
netdev_ops pointer in it's ->init() function but this
leaves the compat method pointers stale.

Add a netdev_resync_ops() and call it from the vlan code.

Any other driver which changes ->netdev_ops after register_netdevice()
will need to call this new function after doing so too.

With help from Patrick McHardy.

Tested-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>