block: remove ->driverfs_dev

Now that all drivers that specify a ->driverfs_dev have been converted
to device_add_disk(), the pointer can be removed from struct gendisk.

Cc: Jens Axboe <axboe@fb.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/nvdimm/bus.c b/drivers/nvdimm/bus.c
index f085f8b..5e4e5c7 100644
--- a/drivers/nvdimm/bus.c
+++ b/drivers/nvdimm/bus.c
@@ -312,7 +312,7 @@
 
 int nvdimm_revalidate_disk(struct gendisk *disk)
 {
-	struct device *dev = disk->driverfs_dev;
+	struct device *dev = disk_to_dev(disk)->parent;
 	struct nd_region *nd_region = to_nd_region(dev->parent);
 	const char *pol = nd_region->ro ? "only" : "write";