x86, microcode, AMD: Fix use after free in free_cache()

list_for_each_entry_reverse() dereferences the iterator, but we already
freed it. I don't see a reason that this has to be done in reverse order
so change it to use list_for_each_entry_safe().
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>

arch/x86/kernel/microcode_amd.c

@@ -150,9 +150,9 @@ static void update_cache(struct ucode_patch *new_patch)
 
 static void free_cache(void)
 {
-	struct ucode_patch *p;
+	struct ucode_patch *p, *tmp;
 
-	list_for_each_entry_reverse(p, &pcache, plist) {
+	list_for_each_entry_safe(p, tmp, &pcache, plist) {
 		__list_del(p->plist.prev, p->plist.next);
 		kfree(p->data);
 		kfree(p);