SUNRPC: Ensure the bvecs are reset when we re-encode the RPC request

The bvec tracks the list of pages, so if the number of pages changes
due to a re-encode, we need to reset the bvec as well.

Fixes: 277e4ab7d530 ("SUNRPC: Simplify TCP receive code by switching...")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: stable@vger.kernel.org # v4.20+
diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
index 70a704c..783748d 100644
--- a/net/sunrpc/xprt.c
+++ b/net/sunrpc/xprt.c
@@ -1041,6 +1041,8 @@ xprt_request_enqueue_receive(struct rpc_task *task)
 
 	if (!xprt_request_need_enqueue_receive(task, req))
 		return;
+
+	xprt_request_prepare(task->tk_rqstp);
 	spin_lock(&xprt->queue_lock);
 
 	/* Update the softirq receive buffer */