selftests/livepatch: fix resource leak in test_klp_syscall init error path

In livepatch_init(), if klp_enable_patch() fails, the previously
created kobject and sysfs file are never cleaned up, causing a
resource leak. Capture the return value and add proper cleanup
on the error path.

Signed-off-by: Rui Qi <qirui.001@bytedance.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://patch.msgid.link/20260604083208.1071428-1-qirui.001@bytedance.com
Signed-off-by: Petr Mladek <pmladek@suse.com>