Adjust KERNEL_ASSERT_UNLOCKED() to not assert during a panic.

KERNEL_ASSERT_UNLOCKED calls _kernel_lock_held() which returns true
if panicstr || db_active which triggers this assert. Workaround this by
checking them before.

This will alter the following Syzkaller reports:
Reported-by: syzbot+169110a0815838ab5940@syzkaller.appspotmail.com
Reported-by: syzbot+3c2eced405b9de6f79c2@syzkaller.appspotmail.com

OK mpi@

sys/sys/systm.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: systm.h,v 1.169 2023/10/17 00:04:02 cheloha Exp $	*/
+/*	$OpenBSD: systm.h,v 1.170 2023/10/30 07:04:36 claudio Exp $	*/
 /*	$NetBSD: systm.h,v 1.50 1996/06/09 04:55:09 briggs Exp $	*/
 
 /*-
@@ -420,7 +420,7 @@ int	_kernel_lock_held(void);
 #define	KERNEL_LOCK()			_kernel_lock()
 #define	KERNEL_UNLOCK()			_kernel_unlock()
 #define	KERNEL_ASSERT_LOCKED()		KASSERT(_kernel_lock_held())
-#define	KERNEL_ASSERT_UNLOCKED()	KASSERT(!_kernel_lock_held())
+#define	KERNEL_ASSERT_UNLOCKED()	KASSERT(panicstr || db_active || !_kernel_lock_held())
 
 #else /* ! MULTIPROCESSOR */