kernel - Do not allow (devfs) devices to be force-unmounted
* Do not force-unmount devfs devices. Even more importantly, don't
force-unmount devfs devices and then try to force-vflush them.
Filesystems have always been responsible for flushing their underlying
block devices.
These situations occur during halt/reboot.
* Fixes at least one issue when hammer's volume-add is used to specify
a block device in /dev which is then added to the root mount. This
created a situation where the /dev vs / unmount order breaks.
* Should also prevent e.g. the console or other vty hangers-on from being
destroyed out from under active descriptors during a halt or reboot.