block: initialize struct block_device in bdev_alloc

Don't play tricks with slab constructors as bdev structures tends to not
get reused very much, and this makes the code a lot less error prone.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>