lkdtm: Add kfence read after free crash type

Add the ability to allocate memory from kfence and trigger a read after
free on that memory to validate that kfence is working properly. This is
used by ChromeOS integration tests to validate that kfence errors can be
collected on user devices and parsed properly.

Cc: Alexander Potapenko <glider@google.com>
Acked-by: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: kasan-dev@googlegroups.com
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20231129214413.3156334-1-swboyd@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/kfence.h

@@ -223,6 +223,8 @@ bool __kfence_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *sla
 
 #else /* CONFIG_KFENCE */
 
+#define kfence_sample_interval	(0)
+
 static inline bool is_kfence_address(const void *addr) { return false; }
 static inline void kfence_alloc_pool_and_metadata(void) { }
 static inline void kfence_init(void) { }