ipv4: take rt_uncached_lock only if needed

Multicast traffic allocates dst with DST_NOCACHE, but dst is
not inserted into rt_uncached_list.

This slowdown multicast workloads on SMP because rt_uncached_lock is
contended.

Change the test before taking the lock to actually check the dst
was inserted into rt_uncached_list.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1263,7 +1263,7 @@ static void ipv4_dst_destroy(struct dst_entry *dst)
 {
 	struct rtable *rt = (struct rtable *) dst;
 
-	if (dst->flags & DST_NOCACHE) {
+	if (!list_empty(&rt->rt_uncached)) {
 		spin_lock_bh(&rt_uncached_lock);
 		list_del(&rt->rt_uncached);
 		spin_unlock_bh(&rt_uncached_lock);