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>