health: Inverting random & sequential IOs

On some controller we need to make sequential IOs before doing the
random one unless unaccurate results returns.

Spreading a great amount of random IOs saturates controller's cache.
Once the fio ended, and even in direct mode, starting the sequential
test for 10sec lead to this situation :

Controller is still mixing the remaining random IOs with the sequential
ones. As a result the sequential one as much more lower since the
controller didn't ended the random one.

Inverting random & sequential one solve this insue since sequential IOs
are completed very quickly and doesn't last into the controller.
health: Flusing disk's buffer before benchmark

Prior any fio benchmarking, let's flush disk's cache to avoid any
performance issue.

As the run_fio() function is called many time, let's be sure that no
more data are located in disk's cache before starting.

That might not be handled by some Raid controller.