rnbd-srv: don't pass a holder for non-exclusive blkdev_get_by_path

Passing a holder to blkdev_get_by_path when FMODE_EXCL isn't set doesn't
make sense, so pass NULL instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20230608110258.189493-14-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>