rxrpc: Fix RTT gathering

Fix RTT information gathering in AF_RXRPC by the following means:

 (1) Enable Rx timestamping on the transport socket with SO_TIMESTAMPNS.

 (2) If the sk_buff doesn't have a timestamp set when rxrpc_data_ready()
     collects it, set it at that point.

 (3) Allow ACKs to be requested on the last packet of a client call, but
     not a service call.  We need to be careful lest we undo:

	bf7d620a
	Author: David Howells <dhowells@redhat.com>
	Date:   Thu Oct 6 08:11:51 2016 +0100
	rxrpc: Don't request an ACK on the last DATA packet of a call's Tx phase

     but that only really applies to service calls that we're handling,
     since the client side gets to send the final ACK (or not).

 (4) When about to transmit an ACK or DATA packet, record the Tx timestamp
     before only; don't update the timestamp afterwards.

 (5) Switch the ordering between recording the serial and recording the
     timestamp to always set the serial number first.  The serial number
     shouldn't be seen referenced by an ACK packet until we've transmitted
     the packet bearing it - so in the Rx path, we don't need the timestamp
     until we've checked the serial number.

Fixes: cf1a6474 ("rxrpc: Add per-peer RTT tracker")
Signed-off-by: David Howells <dhowells@redhat.com>