md: remove mddev_to_conf "helper" macro

Having a macro just to cast a void* isn't really helpful.
I would must rather see that we are simply de-referencing ->private,
than have to know what the macro does.

So open code the macro everywhere and remove the pointless cast.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/linear.h

@@ -24,6 +24,4 @@ struct linear_private_data
 
 typedef struct linear_private_data linear_conf_t;
 
-#define mddev_to_conf(mddev) ((linear_conf_t *) mddev->private)
-
 #endif