kernel: fix filp_open on older kernel's kworker (#205)

On older kernel, kworker missing keyring from init process , and this
keyring is related to FBE , which causes filp_open return ENOKEY or
other errors.To fix this,just install init's keyring to per
kworkers.This works on Kernel 4.4 and 4.9.

kernel/uid_observer.c

@@ -39,6 +39,7 @@ static bool is_uid_exist(uid_t uid, void *data)
 
 static void do_update_uid(struct work_struct *work)
 {
+	KWORKER_INSTALL_KEYRING();
 	struct file *fp = filp_open(SYSTEM_PACKAGES_LIST_PATH, O_RDONLY, 0);
 	if (IS_ERR(fp)) {
 		pr_err("do_update_uid, open " SYSTEM_PACKAGES_LIST_PATH