svcrdma: Catch another Reply chunk overflow case

When space in the Reply chunk runs out in the middle of a segment,
we end up passing a zero-length SGL to rdma_rw_ctx_init(), and it
oopses.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/xprtrdma/svc_rdma_rw.c

@@ -460,6 +460,8 @@ svc_rdma_build_writes(struct svc_rdma_write_info *info,
 		offset += info->wi_seg_off;
 
 		write_len = min(remaining, length - info->wi_seg_off);
+		if (!write_len)
+			goto out_overflow;
 		ctxt = svc_rdma_get_rw_ctxt(rdma,
 					    (write_len >> PAGE_SHIFT) + 2);
 		if (!ctxt)