trace: order samples by time as well as duration

list_time can currently be sorted by duration. Since we read things from the
buffer in buffer order to construct the trace list, the list is already
quasi-sorted by its timestamp key: it is per-cpu sorted, but globally we have
not a lot of guarantees and some samples will get reordered.  It would be
useful to have a list that is strongly sorted as well.

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>