selftests/resctrl: Move run_benchmark() to a more fitting file

resctrlfs.c contains mostly functions that interact in some way with
resctrl FS entries while functions inside resctrl_val.c deal with
measurements and benchmarking.

run_benchmark() is located in resctrlfs.c even though it's purpose
is not interacting with the resctrl FS but to execute cache checking
logic.

Move run_benchmark() to resctrl_val.c just before resctrl_val() that
makes use of run_benchmark(). Make run_benchmark() static since it's
not used between multiple files anymore.

Remove return comment from kernel-doc since the function is type void.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>