Attempting to access a device which has been destroyed, such as a UMASS

Attempting to access a device which has been destroyed, such as a UMASS
device which has been pulled out and detached from its /dev/daX entry,
panics the system.  This occurs because the device might not be immediately
(or ever) removed from the device hash after the device has been destroyed.
It's fields are cleared, leading to a NULL function call.

Fix the panic by installing a dummy cdevsw when the device is destroyed
rather then NULLing it out.

Reported-by: Sascha Wildner <saw@online.de>