cryptodisk: Fix incorrect calculation of start sector
    
    Here dev is a grub_cryptodisk_t and dev->offset is offset in sectors of size
    native to the cryptodisk device. The sector is correctly transformed into
    native grub sector size, but then added to dev->offset which is not
    transformed. It would be nice if the type system would help us with this.
    
    Signed-off-by: Glenn Washburn <development@efficientek.com>
    Reviewed-by: Patrick Steinhardt <ps@pks.im>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>