hfsplus: fix overflow in hfsplus_read_wrapper

For partitions larger than 2TB or at such an offset the hfs wrapper code
in hfsplus might overflow the range representable in a 32-bit
data type. Make sure we use a sector_t for the arithmetics leading to it.

I'm not sure this code can be readed at all as hfs itself never supported
such large volumes.

Signed-off-by: Christoph Hellwig <hch@tuxera.com>