net: Pre-COW metrics for TCP.

TCP is going to record metrics for the connection,
so pre-COW the route metrics at route cache entry
creation time.

This avoids several atomic operations that have to
occur if we COW the metrics after the entry reaches
global visibility.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/flow.h

@@ -48,7 +48,8 @@ struct flowi {
 
 	__u8	proto;
 	__u8	flags;
-#define FLOWI_FLAG_ANYSRC 0x01
+#define FLOWI_FLAG_ANYSRC		0x01
+#define FLOWI_FLAG_PRECOW_METRICS	0x02
 	union {
 		struct {
 			__be16	sport;