block: convert fops.c magic constants to SHIFT_SECTOR

Don't use shifting by a magic number 9 but replace with a more
descriptive SHIFT_SECTOR.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/068782b9f7e97569fb59a99529b23bb17ea4c5e2.1634755800.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>