xprtrdma: Chunk list encoders must not return zero

Clean up, based on code audit: Remove the possibility that the
chunk list XDR encoders can return zero, which would be interpreted
as a NULL.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/fmr_ops.c

@@ -246,6 +246,8 @@ fmr_op_map(struct rpcrdma_xprt *r_xprt, struct rpcrdma_mr_seg *seg,
 	}
 	mw->mw_nents = i;
 	mw->mw_dir = rpcrdma_data_dir(writing);
+	if (i == 0)
+		goto out_dmamap_err;
 
 	if (!ib_dma_map_sg(r_xprt->rx_ia.ri_device,
 			   mw->mw_sg, mw->mw_nents, mw->mw_dir))

net/sunrpc/xprtrdma/frwr_ops.c

@@ -411,6 +411,8 @@ frwr_op_map(struct rpcrdma_xprt *r_xprt, struct rpcrdma_mr_seg *seg,
 	}
 	mw->mw_nents = i;
 	mw->mw_dir = rpcrdma_data_dir(writing);
+	if (i == 0)
+		goto out_dmamap_err;
 
 	dma_nents = ib_dma_map_sg(ia->ri_device,
 				  mw->mw_sg, mw->mw_nents, mw->mw_dir);

net/sunrpc/xprtrdma/rpc_rdma.c

@@ -329,7 +329,7 @@ rpcrdma_encode_read_list(struct rpcrdma_xprt *r_xprt,
 
 	do {
 		n = r_xprt->rx_ia.ri_ops->ro_map(r_xprt, seg, nsegs, false);
-		if (n <= 0)
+		if (n < 0)
 			return ERR_PTR(n);
 
 		*iptr++ = xdr_one;	/* item present */
@@ -397,7 +397,7 @@ rpcrdma_encode_write_list(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req,
 	nchunks = 0;
 	do {
 		n = r_xprt->rx_ia.ri_ops->ro_map(r_xprt, seg, nsegs, true);
-		if (n <= 0)
+		if (n < 0)
 			return ERR_PTR(n);
 
 		iptr = xdr_encode_rdma_segment(iptr, seg);
@@ -462,7 +462,7 @@ rpcrdma_encode_reply_chunk(struct rpcrdma_xprt *r_xprt,
 	nchunks = 0;
 	do {
 		n = r_xprt->rx_ia.ri_ops->ro_map(r_xprt, seg, nsegs, true);
-		if (n <= 0)
+		if (n < 0)
 			return ERR_PTR(n);
 
 		iptr = xdr_encode_rdma_segment(iptr, seg);