kernel diskmbr handling - Detect maxed out slice ds_size field.

kernel diskmbr handling - Detect maxed out slice ds_size field.

If the DOS slice field is found to be maxed out (0xFFFFFFFFU), use the
actual media size for calculations instead of ds_size.  This allows
disks > 2TB to have a conventional slice table and still contain a
disklabel64 which covers the actual size of the media.