selftests: locking: return Kselftest Skip code for skipped tests

When locking 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.

Added an explicit search for ww_mutex module and return skip code if
it isn't found to differentiate between the failure to load the module
condition and module not found condition.

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>