This allows a driver to pass a queuedata member before ->init_hctx is
called.  null_blk currently open codes this logic, but I'd rather have
it in the core to ease future maintainance.
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>