selftests: vDSO: chacha: Correctly skip test if necessary

According to kselftest.h ksft_exit_skip() is not meant to be called when
a plan has already been printed.

Use the recommended function ksft_test_result_skip().

This fixes a bug, where the TAP output would be invalid when skipping:

	TAP version 13
	1..1
	ok 2 # SKIP Not implemented on architecture

The SKIP line should start with "ok 1" as the plan only contains one test.

Fixes: 3b5992e ("selftests: vDSO: unconditionally build chacha test")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/all/20250611-selftests-vdso-fixes-v3-1-e62e37a6bcf5@linutronix.de

Author: t-8ch

tools/testing/selftests/vDSO/vdso_test_chacha.c

@@ -76,7 +76,8 @@ static void reference_chacha20_blocks(uint8_t *dst_bytes, const uint32_t *key, u
 
 void __weak __arch_chacha20_blocks_nostack(uint8_t *dst_bytes, const uint32_t *key, uint32_t *counter, size_t nblocks)
 {
-	ksft_exit_skip("Not implemented on architecture\n");
+	ksft_test_result_skip("Not implemented on architecture\n");
+	ksft_finished();
 }
 
 int main(int argc, char *argv[])