selftests: membarrier: return Kselftest Skip code for skipped tests

When membarrier test is skipped because of unmet dependencies and/or
unsupported configuration, it exits with error which is treated as a
fail by the Kselftest framework. This leads to false negative result
even when the test could not be run.

Change it to return kselftest skip code when a test gets skipped to
clearly report that the test could not be run.

Change it to use ksft_exit_skip() when the test is skipped.

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>