Add device_ref/device_unref calls to prevent a possible use-after-free issue in disk_attach_callback.  Assumes that the struct disk is part of the driver's softc, but this is always true in practice.
Add device_ref/device_unref calls to prevent a possible use-after-free
issue in disk_attach_callback.  Assumes that the struct disk is part
of the driver's softc, but this is always true in practice.

Still other scary use-after-free races lying around here though...

ok jsing@, deraadt@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars