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>