Implement simple ops for umass_disk

The initial IOCTL implementation supports reading disk physical
geometry.
Two additional functions were added. They allow reading/writing raw
data to the disk (default partition).

Submitted by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Sponsored by: Juniper Networks Inc.
Differential Revision: https://reviews.freebsd.org/D4143