Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/kernel/notifier.c b/kernel/notifier.c
index 6196af8..d9f5081 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0-only
 #include <linux/kdebug.h>
 #include <linux/kprobes.h>
 #include <linux/export.h>
@@ -22,6 +23,7 @@
 		struct notifier_block *n)
 {
 	while ((*nl) != NULL) {
+		WARN_ONCE(((*nl) == n), "double register detected");
 		if (n->priority > (*nl)->priority)
 			break;
 		nl = &((*nl)->next);