selftests: efivarfs: return Kselftest Skip code for skipped tests

BugLink: https://bugs.launchpad.net/bugs/1809704

When efivarfs test is skipped because of unmet dependencies and/or
unsupported configuration, it returns 0 which is treated as a pass
by the Kselftest framework. This leads to false positive 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>
(backported from commit 1f0ea958)
[PHLin: return 0 as the framework can't handle ksft_skip]
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

tools/testing/selftests/efivarfs/efivarfs.sh

@@ -3,6 +3,9 @@
 efivarfs_mount=/sys/firmware/efi/efivars
 test_guid=210be57c-9849-4fc7-a635-e6382d1aec27
 
+# Kselftest framework requirement - SKIP code is 4.
+ksft_skip=4
+
 check_prereqs()
 {
 	local msg="skip all tests:"