net: fix __dst_destroy_metrics_generic()

dst_default_metrics is readonly, we dont want to kfree() it later.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/dst.c b/net/core/dst.c
index 81a4fa1..9ccca03 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -315,7 +315,7 @@
 {
 	unsigned long prev, new;
 
-	new = (unsigned long) dst_default_metrics;
+	new = ((unsigned long) dst_default_metrics) | DST_METRICS_READ_ONLY;
 	prev = cmpxchg(&dst->_metrics, old, new);
 	if (prev == old)
 		kfree(__DST_METRICS_PTR(old));