Made kernel accept IPv6 routes with IPv4-mapped address as next-hop.

It is possible to configure IP interfaces with IPv4-mapped addresses, and
one can add IPv6 routes for IPv4-mapped destinations/prefixes, yet prior
to this fix the kernel returned an EINVAL when attempting to add an IPv6
route with an IPv4-mapped address as a nexthop/gateway.

RFC 4798 (a proposed standard RFC) uses IPv4-mapped addresses as nexthops,
thus in order to support that type of address configuration the kernel
needs to allow IPv4-mapped addresses as nexthops.
Signed-off-by: Erik Nordmark <nordmark@arista.com>
Signed-off-by: Bob Gilligan <gilligan@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>