Break the following implementation of panic(3):

#!bin/sh

	# Original version of this by Michael Reifenberger
	# <root@nihil.plaut.de>.

	mdconfig -d -u 11 >/dev/null 2>&1
	dd if=/dev/zero of=zz bs=1m count=1

	while :
	do
		mdconfig -a -t vnode -f zz -u 11
		fdisk -f - -iv /dev/md11 <<EOF1
		g c1 h64 s32
		p 1 165 0 2048
		a 1
	EOF1
		mdconfig -d -u 11
	done

Garbage pointers in __si_u were not cleared by destroy_dev().  Not
clearing si_disk made the above fatal because the disk layer uses
si_disk as a flag to indicate that the dev_t has been completely
initialized.  disk_destroy() clears si_disk for the parent dev_t
but doesn't get called for children.

Not fixed:
- setting the undocumented sysctl debug.free_devt should cause more
  complete destruction of the dev_t including clearing of __si_u, but
  actually causes the above to panic a little earlier.
- the loop leaks 10 memory allocations per iteration (4 DEVFS, 2 devbuf
  and 4 dev_t).

Reviewed by:	timeout by MAINTAINER after 3 months

sys/kern/kern_conf.c

@@ -398,6 +398,7 @@ destroy_dev(dev_t dev)
 	dev->si_drv1 = 0;
 	dev->si_drv2 = 0;
 	dev->si_devsw = 0;
+	bzero(&dev->__si_u, sizeof(dev->__si_u));
 	dev->si_flags &= ~SI_NAMED;
 	dev->si_flags &= ~SI_ALIAS;
 	freedev(dev);