Added boilerplate code for ext4 fs.

Initialized BUILD with license
Mount is still unimplemented and is not meant to be
part of this CL. Rest of the fs interface is implemented.
Referenced the Linux kernel appropriately when needed

PiperOrigin-RevId: 249741997
Change-Id: Id1e4c7c9e68b3f6946da39896fc6a0c3dcd7f98c

pkg/sentry/fs/ext4/BUILD

@@ -0,0 +1,14 @@
+package(licenses = ["notice"])
+
+load("//tools/go_stateify:defs.bzl", "go_library")
+
+go_library(
+    name = "ext4",
+    srcs = ["fs.go"],
+    importpath = "gvisor.googlesource.com/gvisor/pkg/sentry/fs/ext4",
+    visibility = ["//pkg/sentry:internal"],
+    deps = [
+        "//pkg/sentry/context",
+        "//pkg/sentry/fs",
+    ],
+)

pkg/sentry/fs/ext4/fs.go

@@ -0,0 +1,61 @@
+// Copyright 2019 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 ext4 implements the ext4 filesystem.
+package ext4
+
+import (
+	"gvisor.googlesource.com/gvisor/pkg/sentry/context"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/fs"
+)
+
+// filesystem implements fs.Filesystem for ext4.
+//
+// +stateify savable
+type filesystem struct{}
+
+func init() {
+	fs.RegisterFilesystem(&filesystem{})
+}
+
+// FilesystemName is the name under which the filesystem is registered.
+// Name matches fs/ext4/super.c:ext4_fs_type.name.
+const FilesystemName = "ext4"
+
+// Name is the name of the file system.
+func (*filesystem) Name() string {
+	return FilesystemName
+}
+
+// AllowUserMount prohibits users from using mount(2) with this file system.
+func (*filesystem) AllowUserMount() bool {
+	return false
+}
+
+// AllowUserList prohibits this filesystem to be listed in /proc/filesystems.
+func (*filesystem) AllowUserList() bool {
+	return false
+}
+
+// Flags returns properties of the filesystem.
+//
+// In Linux, ext4 returns FS_REQUIRES_DEV. See fs/ext4/super.c
+func (*filesystem) Flags() fs.FilesystemFlags {
+	return fs.FilesystemRequiresDev
+}
+
+// Mount returns the root inode of the ext4 fs.
+func (f *filesystem) Mount(ctx context.Context, device string, flags fs.MountSourceFlags, data string, cgroupsInt interface{}) (*fs.Inode, error) {
+	panic("unimplemented")
+}