block: implement and use {disk|part}_to_dev()

Implement {disk|part}_to_dev() and use them to access generic device
instead of directly dereferencing {disk|part}->dev.  To make sure no
user is left behind, rename generic devices fields to __dev.

This is in preparation of unifying partition 0 handling with other
partitions.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 1778e4a..7b33512 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -403,7 +403,7 @@
 	BUG_ON(lo->magic != LO_MAGIC);
 
 	lo->pid = current->pid;
-	ret = sysfs_create_file(&lo->disk->dev.kobj, &pid_attr.attr);
+	ret = sysfs_create_file(&disk_to_dev(lo->disk)->kobj, &pid_attr.attr);
 	if (ret) {
 		printk(KERN_ERR "nbd: sysfs_create_file failed!");
 		return ret;
@@ -412,7 +412,7 @@
 	while ((req = nbd_read_stat(lo)) != NULL)
 		nbd_end_request(req);
 
-	sysfs_remove_file(&lo->disk->dev.kobj, &pid_attr.attr);
+	sysfs_remove_file(&disk_to_dev(lo->disk)->kobj, &pid_attr.attr);
 	return 0;
 }