ip6_tunnel: Fix the type of functions

Functions ip6_tnl_change, ip6_tnl_update and ip6_tnl0_update do always
return 0, change the type of functions to void.

Signed-off-by: Hongbin Wang <wh_bin@126.com>
Signed-off-by: David S. Miller <davem@davemloft.net>