ip6mr: change the prototype of ip6_mr_forward().

This patch changes the prototpye of the ip6_mr_forward() method to return void
instead of int.

The ip6_mr_forward() method always returns 0; moreover, the return value of this
method is not checked anywhere.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>