selftests: membarrier: use ksft_* var arg msg api

Use ksft_* var arg msg to include strerror() info. in test output. Remove
redundant SKIP/FAIL/PASS logic as it is no longer needed with ksft_ api.
Improve test output to be consistent and clear.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>