net: ping6: support packet timestamping

Nothing prevents the user from requesting timestamping
on ping6 sockets, yet timestamps are not going to be reported.
Plumb the flags through.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>