ipv4: Dirty less cache lines in route caching paths.

Don't bother incrementing dst->__use and setting dst->lastuse,
they are completely pointless and just slow things down.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1443,7 +1443,7 @@ static int __mkroute_input(struct sk_buff *skb,
 		if (!(flags & RTCF_DIRECTSRC) && !itag) {
 			rth = FIB_RES_NH(*res).nh_rth_input;
 			if (rt_cache_valid(rth)) {
-				dst_use(&rth->dst, jiffies);
+				dst_hold(&rth->dst);
 				goto out;
 			}
 			do_cache = true;
@@ -1619,7 +1619,7 @@ out:	return err;
 		if (!(flags & RTCF_DIRECTSRC) && !itag) {
 			rth = FIB_RES_NH(res).nh_rth_input;
 			if (rt_cache_valid(rth)) {
-				dst_use(&rth->dst, jiffies);
+				dst_hold(&rth->dst);
 				goto set_and_out;
 			}
 			do_cache = true;
@@ -1789,7 +1789,7 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
 		if (!fnhe) {
 			rth = FIB_RES_NH(*res).nh_rth_output;
 			if (rt_cache_valid(rth)) {
-				dst_use(&rth->dst, jiffies);
+				dst_hold(&rth->dst);
 				return rth;
 			}
 		}