selftests: vDSO: vdso_test_abi: Correctly skip whole test with missing vDSO

If AT_SYSINFO_EHDR is missing the whole test needs to be skipped.
Currently this results in the following output:

	TAP version 13
	1..16
	# AT_SYSINFO_EHDR is not present!

This output is incorrect, as "1..16" still requires the subtest lines to
be printed, which isn't done however.

Switch to the correct skipping functions, so the output now correctly
indicates that no subtests are being run:

	TAP version 13
	1..0 # SKIP AT_SYSINFO_EHDR is not present!

Fixes: 693f5ca ("kselftest: Extend vDSO selftest")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250812-vdso-tests-fixes-v2-2-90f499dd35f8@linutronix.de

Author: t-8ch

tools/testing/selftests/vDSO/vdso_test_abi.c

@@ -182,12 +182,11 @@ int main(int argc, char **argv)
 	unsigned long sysinfo_ehdr = getauxval(AT_SYSINFO_EHDR);
 
 	ksft_print_header();
-	ksft_set_plan(VDSO_TEST_PLAN);
 
-	if (!sysinfo_ehdr) {
-		ksft_print_msg("AT_SYSINFO_EHDR is not present!\n");
-		return KSFT_SKIP;
-	}
+	if (!sysinfo_ehdr)
+		ksft_exit_skip("AT_SYSINFO_EHDR is not present!\n");
+
+	ksft_set_plan(VDSO_TEST_PLAN);
 
 	version = versions[VDSO_VERSION];
 	name = (const char **)&names[VDSO_NAMES];