Fixed the build on Linux.

internal/buffer/out_message_linux.go

+// Copyright 2015 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package buffer
+
+// The maximum read size that we expect to ever see from the kernel, used for
+// calculating the size of out messages.
+//
+// For 4 KiB pages, this is 128 KiB (cf. https://goo.gl/HOiEYo)
+const MaxReadSize = 1 << 17