block: re-arrange the struct block_device fields for better layout

This moves struct device out-of-line as it's just used at open/close
time, so we can keep some of the commonly used fields closer together.
On a standard setup, it also reduces the size from 864 bytes to 848
bytes. Yes, struct device is a pig...

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