kassert: Remove KASAN marking from DEBUG_POISON_POINTER

While this approach works for trapping reads of an uninitialized
pointer, it means that any attempt to store to the variable triggers a
KASAN report, which is not what we want.  Simply remove the kasan_mark()
call.  KMSAN will catch these kinds of bugs automatically anyway.

Reported by:	syzbot+c5e279db5763b58e8d69@syzkaller.appspotmail.com
MFC after:	1 week

sys/sys/kassert.h

@@ -38,8 +38,7 @@ extern const char *panicstr;	/* panic message */
 #define	KERNEL_PANICKED()	__predict_false(panicstr != NULL)
 
 /*
- * Trap accesses going through a pointer. Moreover if kasan is available trap
- * reading the pointer itself.
+ * Trap accesses going through a pointer.
  *
  * Sample usage: you have a struct with numerous fields and by API contract
  * only some of them get populated, even if the implementation temporary writes
@@ -61,7 +60,6 @@ extern caddr_t poisoned_buf;
 
 #define DEBUG_POISON_POINTER(x) ({				\
 	x = (void *)(DEBUG_POISON_POINTER_VALUE);		\
-	kasan_mark(&x, 0, sizeof(x), KASAN_GENERIC_REDZONE);	\
 })
 
 #else