block: unexport read_dev_sector and put_dev_sector

read_dev_sector and put_dev_sector are now only used by the partition
parsing code.  Remove the export for read_dev_sector and merge it into
the only caller.  Clean the mess up a bit by using goto labels and
the SECTOR_SHIFT constant.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>