ipv6: Allow IPv4-mapped address as next-hop

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>

net/ipv6/route.c

@@ -2000,8 +2000,11 @@ static struct rt6_info *ip6_route_info_create(struct fib6_config *cfg)
 			   It is very good, but in some (rare!) circumstances
 			   (SIT, PtP, NBMA NOARP links) it is handy to allow
 			   some exceptions. --ANK
+			   We allow IPv4-mapped nexthops to support RFC4798-type
+			   addressing
 			 */
-			if (!(gwa_type & IPV6_ADDR_UNICAST))
+			if (!(gwa_type & (IPV6_ADDR_UNICAST |
+					  IPV6_ADDR_MAPPED)))
 				goto out;
 
 			if (cfg->fc_table) {