rxrpc: Record probes after transmission and reduce number of time-gets

Move the recording of a successfully transmitted DATA or ACK packet that
will provide RTT probing to after the transmission.  With the I/O thread
model, this can be done because parsing of the responding ACK can no longer
race with the post-transmission code.

Move the various timeout-settings done after successfully transmitting a
DATA packet into rxrpc_tstamp_data_packets() and eliminate a number of
calls to get the current time.

As a consequence we no longer need to cancel a proposed RTT probe on
transmission failure.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org