selftests: sigaltstack: do not use ksft_exit_skip after ksft_set_plan

Calling ksft_exit_skip after ksft_set_plan results in executing fewer tests
than planned. Use ksft_test_result_skip when possible, or just bail out if
memory corruption is detected.

Signed-off-by: Paolo Bonzini <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/sigaltstack/sas.c

···
 	swapcontext(&sc, &uc);
 	ksft_print_msg("%s\n", p->msg);
 	if (!p->flag) {
-		ksft_exit_skip("[RUN]\tAborting\n");
+		ksft_exit_fail_msg("[RUN]\tAborting\n");
 		exit(EXIT_FAILURE);
 	}
 }
···
 	err = sigaltstack(&stk, NULL);
 	if (err) {
 		if (errno == EINVAL) {
-			ksft_exit_skip(
+			ksft_test_result_skip(
 				"[NOTE]\tThe running kernel doesn't support SS_AUTODISARM\n");
 			/*
 			 * If test cases for the !SS_AUTODISARM variant were