packet: fix second argument of sock_tx_timestamp()

This patch fixes an issue that a syscall (e.g. sendto syscall) cannot
work correctly. Since the sendto syscall doesn't have msg_control buffer,
the sock_tx_timestamp() in packet_snd() cannot work correctly because
the socks.tsflags is set to 0.
So, this patch sets the socks.tsflags to sk->sk_tsflags as default.

Fixes: c14ac945

 ("sock: enable timestamping using control messages")
Reported-by: Kazuya Mizuguchi <kazuya.mizuguchi.ks@renesas.com>
Reported-by: Keita Kobayashi <keita.kobayashi.ym@renesas.com>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>