block: move capacity from disk to part0

Move disk->capacity to part0->nr_sects and convert all users who
directly accessed the field to use {get|set}_capacity().  This is done
early to allow the __dev field to be moved.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/drivers/block/aoe/aoedev.c b/drivers/block/aoe/aoedev.c
index a1d813a..6a8038d 100644
--- a/drivers/block/aoe/aoedev.c
+++ b/drivers/block/aoe/aoedev.c
@@ -91,7 +91,7 @@
 	}
 
 	if (d->gd)
-		d->gd->capacity = 0;
+		set_capacity(d->gd, 0);
 
 	d->flags &= ~DEVFL_UP;
 }