vti6: delete unneeded call to netdev_priv

Netdev_priv is an accessor function, and has no purpose if its result is
not used.

A simplified version of the semantic match that fixes this problem is as
follows: (http://coccinelle.lip6.fr/

)

// <smpl>
@@ local idexpression x; @@
-x = netdev_priv(...);
... when != x
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>