sunvdc: compute vdisk geometry from capacity

The LDom diskserver doesn't return reliable geometry data. In addition,
the types for all fields in the vio_disk_geom are u16, which were being
truncated in the cast into the u8's of the Linux struct hd_geometry.

Modify vdc_getgeo() to compute the geometry from the disk's capacity in a
manner consistent with xen-blkfront::blkif_getgeo().

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>