selftests: memory-hotplug: return Kselftest Skip code for skipped tests

When memory-hotplug test is skipped because of unmet dependencies and/or
unsupported configuration, it returns non-zero value hich 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.

Kselftest framework SKIP code is 4 and the framework prints appropriate
messages to indicate that the test is skipped.

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