/* adjusts the offset and number of sectors in the destination pda so that * it covers at most the region of the SU covered by the source PDA. This * is exclusively a restriction: the number of sectors indicated by the * target PDA can only shrink. * * For example: s = sectors within SU indicated by source PDA * d = sectors within SU indicated by dest PDA * r = results, stored in dest PDA * * |--------------- one stripe unit ---------------------| * | sssssssssssssssssssssssssssssssss | * | ddddddddddddddddddddddddddddddddddddddddddddd | * | rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr | * * Another example: * * |--------------- one stripe unit ---------------------| * | sssssssssssssssssssssssssssssssss | * | ddddddddddddddddddddddd | * | rrrrrrrrrrrrrrrr | * */