svcrdma: Retain the page backing rq_res.head[0].iov_base

svc_rdma_sendto() now waits for the NIC hardware to finish with
the pages backing rq_res. We still have to release the page array
in some cases, but now it's always safe to immediately re-use the
page backing rq_res's head buffer.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/xprtrdma/svc_rdma_sendto.c

@@ -957,6 +957,11 @@ int svc_rdma_sendto(struct svc_rqst *rqstp)
 	ret = svc_rdma_send_reply_msg(rdma, sctxt, rctxt, rqstp);
 	if (ret < 0)
 		goto err1;
+
+	/* Prevent svc_xprt_release() from releasing the page backing
+	 * rq_res.head[0].iov_base. It's no longer being accessed by
+	 * the I/O device. */
+	rqstp->rq_respages++;
 	return 0;
 
  err2: