Revert "xen/blkback: Don't trust the handle from the frontend."

This reverts commit 01c681d4 upstream
(ef56ca64 in this tree), as it wasn't
supposed to have been applied to the stable tree.

Cc: Jan Beulich <jbeulich@suse.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/block/xen-blkback/blkback.c

@@ -881,6 +881,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 		goto fail_response;
 	}
 
+	preq.dev           = req->u.rw.handle;
 	preq.sector_number = req->u.rw.sector_number;
 	preq.nr_sects      = 0;