Drop various include not actually used in genhd.h itself, and
move the remaning includes closer together.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20210920123328.1399408-15-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>