ovl: update ctx->pos on impure dir iteration

This fixes a regression with readdir of impure dir in overlayfs
that is shared to VM via 9p fs.

Reported-by: Miguel Bernal Marin <miguel.bernal.marin@linux.intel.com>
Fixes: 4edb83bb10 ("ovl: constant d_ino for non-merge dirs")
Cc: <stable@vger.kernel.org> #4.14
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Miguel Bernal Marin <miguel.bernal.marin@linux.intel.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/readdir.c

@@ -663,7 +663,10 @@ static int ovl_iterate_real(struct file *file, struct dir_context *ctx)
 			return PTR_ERR(rdt.cache);
 	}
 
-	return iterate_dir(od->realfile, &rdt.ctx);
+	err = iterate_dir(od->realfile, &rdt.ctx);
+	ctx->pos = rdt.ctx.pos;
+
+	return err;
 }