net-timestamp: sock_tx_timestamp() fix

sock_tx_timestamp() should not ignore initial *tx_flags value, as TCP
stack can store SKBTX_SHARED_FRAG in it.

Also first argument (struct sock *) can be const.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 4ed2d765dfac ("net-timestamp: TCP timestamping")
Cc: Willem de Bruijn <willemb@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/sock.h b/include/net/sock.h
index 52fe0bc..38805fa 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2199,9 +2199,11 @@
 /**
  * sock_tx_timestamp - checks whether the outgoing packet is to be time stamped
  * @sk:		socket sending this packet
- * @tx_flags:	filled with instructions for time stamping
+ * @tx_flags:	completed with instructions for time stamping
+ *
+ * Note : callers should take care of initial *tx_flags value (usually 0)
  */
-void sock_tx_timestamp(struct sock *sk, __u8 *tx_flags);
+void sock_tx_timestamp(const struct sock *sk, __u8 *tx_flags);
 
 /**
  * sk_eat_skb - Release a skb if it is no longer needed