selftests/resctrl: Cleanup benchmark argument parsing

Benchmark argument is handled by custom argument parsing code which is
more complicated than it needs to be.

Process benchmark argument within the normal getopt() handling and drop
unnecessary ben_ind and has_ben variables. When -b is given, terminate
the argument processing as -b consumes all remaining arguments.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: "Wieczor-Retman, Maciej" <maciej.wieczor-retman@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>