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>

tools/testing/selftests/locking/ww_mutex.sh

 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
+
+# Kselftest framework requirement - SKIP code is 4.
+ksft_skip=4
+
 # Runs API tests for struct ww_mutex (Wait/Wound mutexes)
+if ! /sbin/modprobe -q -n test-ww_mutex; then
+	echo "ww_mutex: module test-ww_mutex is not found [SKIP]"
+	exit $ksft_skip
+fi
 
 if /sbin/modprobe -q test-ww_mutex; then
        /sbin/modprobe -q -r test-ww_mutex