xprtrdma: Cap size of callback buffer resources

When the inline threshold size is set to large values (say, 32KB)
any NFSv4.1 CB request from the server gets a reply with status
NFS4ERR_RESOURCE.

Looks like there are some upper layer assumptions about the maximum
size of a reply (for example, in process_op). Cap the size of the
NFSv4 client's reply resources at a page.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/backchannel.c

@@ -55,7 +55,8 @@ static int rpcrdma_bc_setup_rqst(struct rpcrdma_xprt *r_xprt,
 	if (IS_ERR(rb))
 		goto out_fail;
 	req->rl_sendbuf = rb;
-	xdr_buf_init(&rqst->rq_snd_buf, rb->rg_base, size);
+	xdr_buf_init(&rqst->rq_snd_buf, rb->rg_base,
+		     min_t(size_t, size, PAGE_SIZE));
 	rpcrdma_set_xprtdata(rqst, req);
 	return 0;
 
@@ -191,6 +192,7 @@ size_t xprt_rdma_bc_maxpayload(struct rpc_xprt *xprt)
 	size_t maxmsg;
 
 	maxmsg = min_t(unsigned int, cdata->inline_rsize, cdata->inline_wsize);
+	maxmsg = min_t(unsigned int, maxmsg, PAGE_SIZE);
 	return maxmsg - RPCRDMA_HDRLEN_MIN;
 }