static int async_io(struct dm_io_client *client, unsigned int num_regions,
		    struct dm_io_region *where, blk_opf_t opf,
		    struct dpages *dp, io_notify_fn fn, void *context)
static int async_io(struct dm_io_client *client, unsigned int num_regions,
		    struct dm_io_region *where, blk_opf_t opf,
		    struct dpages *dp, io_notify_fn fn, void *context,
		    unsigned short ioprio)