selftests/resctrl: Ensure the benchmark commands fits to its array

Benchmark command is copied into an array in the stack. The array is
BENCHMARK_ARGS items long but the command line could try to provide a
longer command. Argument size is also fixed by BENCHMARK_ARG_SIZE (63
bytes of space after fitting the terminating \0 character) and user
could have inputted argument longer than that.

Return error in case the benchmark command does not fit to the space
allocated for it.

Fixes: ecdbb911

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