kselftest/vDSO: Use ksft_print_msg() rather than printf in vdso_test_abi

There are a couple of raw printf() calls in vdso_test_abi which result in
non KTAP conforment output such as

   [vDSO kselftest] VDSO_VERSION: LINUX_2.6

Convert them to use ksft_print_msg() so that they don't cause confusion for
parsers.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>