block: switch ->getgeo() to struct gendisk

Instances are happier that way and it makes more sense anyway -
the only part of the result that is related to partition we are given
is the start sector, and that has been filled in by the caller.

Everything else is a function of the disk.  Only one instance
(DASD) is ever looking at anything other than bdev->bd_disk and
that one is trivial to adjust.

Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>