osdep/freebsd: Fix partition calculation for EBR entries

osdep/freebsd: Fix partition calculation for EBR entries

For EBR partitions, "start" is the relative starting sector of the EBR
header itself, whereas "offset" is the relative starting byte of the
partition's contents, excluding the EBR header and any padding. Thus we
must use "offset", and divide by the sector size to convert to sectors.

Fixes Debian bug #923253.

Signed-off-by: James Clarke <jrtc27@jrtc27.com>
Reviewed-by: Colin Watson <cjwatson@ubuntu.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>