[DCCP]: Handle timestamps on Request/Response exchange separately

[DCCP]: Handle timestamps on Request/Response exchange separately

In DCCP, timestamps can occur on packets anytime, CCID3 uses a timestamp(/echo) on the Request/Response
exchange. This patch addresses the following situation:
	* timestamps are recorded on the listening socket;
	* Responses are sent from dccp_request_sockets;
	* suppose two connections reach the listening socket with very small time in between:
	* the first timestamp value gets overwritten by the second connection request.

This is not really good, so this patch separates timestamps into
 * those which are received by the server during the initial handshake (on dccp_request_sock);
 * those which are received by the client or the client after connection establishment.

As before, a timestamp of 0 is regarded as indicating that no (meaningful) timestamp has been
received (in addition, a warning message is printed if hosts send 0-valued timestamps).

The timestamp-echoing now works as follows:
 * when a timestamp is present on the initial Request, it is placed into dreq, due to the
   call to dccp_parse_options in dccp_v{4,6}_conn_request;
 * when a timestamp is present on the Ack leading from RESPOND => OPEN, it is copied over
   from the request_sock into the child cocket in dccp_create_openreq_child;
 * timestamps received on an (established) dccp_sock are treated as before.

Since Elapsed Time is measured in hundredths of milliseconds (13.2), the new dccp_timestamp()
function is used, as it is expected that the time between receiving the timestamp and
sending the timestamp echo will be very small against the wrap-around time. As a byproduct,
this allows smaller timestamping-time fields.

Furthermore, inserting the Timestamp Echo option has been taken out of the block starting with
'!dccp_packet_without_ack()', since Timestamp Echo can be carried on any packet (5.8 and 13.3).

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>