block: move blk_queue_get_max_sectors to blk.h

blk_queue_get_max_sectors is private to the block layer, so move it out
of blkdev.h.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220614090934.570632-7-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>