[BLOCK] Move sector_div() from blkdev.h to kernel.h

We need it even if CONFIG_BLOCK is disabled, so move it outside of
the block layer include system.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --cc include/linux/blkdev.h
Simple merge

diff --cc include/linux/kernel.h
Simple merge