vhost-vdpa: Fix the wrong input in config_cb

Fix the wrong input in for config_cb. In function vhost_vdpa_config_cb,
the input cb.private was used as struct vhost_vdpa, so the input was
wrong here, fix this issue

Fixes: 776f3950 ("vhost_vdpa: Support config interrupt in vdpa")
Signed-off-by: Cindy Lu <lulu@redhat.com>
Link: https://lore.kernel.org/r/20210929090933.20465-1-lulu@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/vdpa.c

@@ -322,7 +322,7 @@ static long vhost_vdpa_set_config_call(struct vhost_vdpa *v, u32 __user *argp)
 	struct eventfd_ctx *ctx;
 
 	cb.callback = vhost_vdpa_config_cb;
-	cb.private = v->vdpa;
+	cb.private = v;
 	if (copy_from_user(&fd, argp, sizeof(fd)))
 		return  -EFAULT;