selftests: vm: return Kselftest Skip code for skipped tests

When vm 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.

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>
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>