drbd: remove ->this_bdev

DRBD keeps a block device open just to get and set the capacity from
it.  Switch to primarily using the disk capacity as intended by the
block layer, and sync it to the bdev using revalidate_disk_size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>