virtio_config: fix len calculation of config elements

Rusty,

This patch is a prereq for the virtio_blk blocksize patch, please apply it
first.

Adding an u32 value to the virtio_blk_config unconvered a small bug the config
space defintions:
v is a pointer, to we have to use sizeof(*v) instead of sizeof(v).

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
index 50db245..71d6c102 100644
--- a/include/linux/virtio_config.h
+++ b/include/linux/virtio_config.h
@@ -99,7 +99,7 @@
  * The return value is -ENOENT if the feature doesn't exist.  Otherwise
  * the config value is copied into whatever is pointed to by v. */
 #define virtio_config_val(vdev, fbit, offset, v) \
-	virtio_config_buf((vdev), (fbit), (offset), (v), sizeof(v))
+	virtio_config_buf((vdev), (fbit), (offset), (v), sizeof(*v))
 
 static inline int virtio_config_buf(struct virtio_device *vdev,
 				    unsigned int fbit,