rxrpc: Adjust the rxrpc_rtt_rx tracepoint

Adjust the rxrpc_rtt_rx tracepoint in the following ways:

 (1) Display the collected RTT sample in the rxrpc_rtt_rx trace.

 (2) Move the division of srtt by 8 to the TP_printk() rather doing it
     before invoking the trace point.

 (3) Display the min_rtt value.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>