net/ipv6: Move gateway validation into helper

Move existing code to validate nexthop into a helper. Follow on patch
adds support for nexthops marked with onlink, and this helper keeps
the complexity of ip6_route_info_create in check.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>