ipv6: Check dest prefix length on original route not copied one in rt6_alloc_cow().

After commit 8e2ec639 ("ipv6: don't
use inetpeer to store metrics for routes.") the test in rt6_alloc_cow()
for setting the ANYCAST flag is now wrong.

'rt' will always now have a plen of 128, because it is set explicitly
to 128 by ip6_rt_copy.

So to restore the semantics of the test, check the destination prefix
length of 'ort'.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -728,7 +728,7 @@ static struct rt6_info *rt6_alloc_cow(const struct rt6_info *ort,
 		int attempts = !in_softirq();
 
 		if (!(rt->rt6i_flags&RTF_GATEWAY)) {
-			if (rt->rt6i_dst.plen != 128 &&
+			if (ort->rt6i_dst.plen != 128 &&
 			    ipv6_addr_equal(&ort->rt6i_dst.addr, daddr))
 				rt->rt6i_flags |= RTF_ANYCAST;
 			ipv6_addr_copy(&rt->rt6i_gateway, daddr);