selftests/resctrl: Fix resctrl_tests' return code to work with selftest framework

In kselftest framework, if a sub test can not run by some reasons,
the test result should be marked as SKIP rather than FAIL.
Return KSFT_SKIP(4) instead of KSFT_FAIL(1) if resctrl_tests is not run
as root or it is run on a test environment which does not support resctrl.

- ksft_exit_fail_msg(): returns KSFT_FAIL(1)
- ksft_exit_skip(): returns KSFT_SKIP(4)

Reviewed-by: Shuah Khan <[e-mail]>
Reviewed-by: Reinette Chatre <[e-mail]>
Reviewed-by: Fenghua Yu <[e-mail]>
Signed-off-by: Shaopeng Tan <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/resctrl/resctrl_tests.c

···
 	 * 2. We execute perf commands
 	 */
 	if (geteuid() != 0)
-		return ksft_exit_fail_msg("Not running as root, abort testing.\n");
+		return ksft_exit_skip("Not running as root. Skipping...\n");
 
 	if (has_ben) {
 		/* Extract benchmark command from command line. */
···
 	sprintf(bm_type, "fill_buf");
 
 	if (!check_resctrlfs_support())
-		return ksft_exit_fail_msg("resctrl FS does not exist\n");
+		return ksft_exit_skip("resctrl FS does not exist. Enable X86_CPU_RESCTRL config option.\n");
 
 	filter_dmesg();