osdep/linux: Convert partition start to disk sector length

osdep/linux: Convert partition start to disk sector length

When reading data off a disk, sector values are based on the disk sector
length.

Within grub_util_fd_open_device(), the start of the partition was taken
directly from grub's partition information structure, which uses the
internal sector length (currently 512b), but never transformed to the
disk's sector length.

Subsequent calculations were all wrong for devices that have a diverging
sector length and the functions eventually skipped to the wrong stream
location, reading invalid data.

Signed-off-by: Mihai Moldovan <ionic@ionic.de>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>