Support VFS2 save/restore.

Inode number consistency checks are now skipped in save/restore tests for
reasons described in greatest detail in StatTest.StateDoesntChangeAfterRename.
They pass in VFS1 due to the bug described in new test case
SimpleStatTest.DifferentFilesHaveDifferentDeviceInodeNumberPairs.

Fixes #1663

PiperOrigin-RevId: 338776148

pkg/sentry/fsimpl/devtmpfs/BUILD

@@ -4,7 +4,10 @@ licenses(["notice"])
 
 go_library(
     name = "devtmpfs",
-    srcs = ["devtmpfs.go"],
+    srcs = [
+        "devtmpfs.go",
+        "save_restore.go",
+    ],
     visibility = ["//pkg/sentry:internal"],
     deps = [
         "//pkg/abi/linux",

pkg/sentry/fsimpl/devtmpfs/save_restore.go

@@ -0,0 +1,23 @@
+// Copyright 2020 The gVisor Authors.
+//
+// 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 devtmpfs
+
+// afterLoad is invoked by stateify.
+func (fst *FilesystemType) afterLoad() {
+	if fst.fs != nil {
+		// Ensure that we don't create another filesystem.
+		fst.initOnce.Do(func() {})
+	}
+}