ipv6 route: use err pointers instead of returning pointer by reference

ipv6 route: use err pointers instead of returning pointer by reference

This patch makes ip6_route_info_create return err pointer instead of
returning the rt pointer by reference as suggested  by Dave

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>