KVM: selftests: Close opened file descriptor in stable_tsc_check_supported()

Close the "current_clocksource" file descriptor before returning or exiting
from stable_tsc_check_supported() in vmx_nested_tsc_scaling_test.
Signed-off-by: Hao Ge <gehao@kylinos.cn>
Reviewed-by: Vipin Sharma <vipinsh@google.com>
Link: https://lore.kernel.org/r/20230405101350.259000-1-gehao@kylinos.cnSigned-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/x86_64/vmx_nested_tsc_scaling_test.c

@@ -126,12 +126,16 @@ static void stable_tsc_check_supported(void)
 		goto skip_test;
 
 	if (fgets(buf, sizeof(buf), fp) == NULL)
-		goto skip_test;
+		goto close_fp;
 
 	if (strncmp(buf, "tsc", sizeof(buf)))
-		goto skip_test;
+		goto close_fp;
 
+	fclose(fp);
 	return;
+
+close_fp:
+	fclose(fp);
 skip_test:
 	print_skip("Kernel does not use TSC clocksource - assuming that host TSC is not stable");
 	exit(KSFT_SKIP);