net: Document dst->obsolete better.

Add a big comment explaining how the field works, and use defines
instead of magic constants for the values assigned to it.

Suggested by Joe Perches.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 84f6564..cf02cb9 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -281,7 +281,7 @@
 					     struct fib6_table *table)
 {
 	struct rt6_info *rt = dst_alloc(&net->ipv6.ip6_dst_ops, dev,
-					0, 0, flags);
+					0, DST_OBSOLETE_NONE, flags);
 
 	if (rt) {
 		struct dst_entry *dst = &rt->dst;
@@ -985,7 +985,7 @@
 	struct rt6_info *rt, *ort = (struct rt6_info *) dst_orig;
 	struct dst_entry *new = NULL;
 
-	rt = dst_alloc(&ip6_dst_blackhole_ops, ort->dst.dev, 1, 0, 0);
+	rt = dst_alloc(&ip6_dst_blackhole_ops, ort->dst.dev, 1, DST_OBSOLETE_NONE, 0);
 	if (rt) {
 		new = &rt->dst;