fusefronted: close race between mknod and chown

If the user manages to replace the directory with
a symlink at just the right time, we could be tricked
into chown'ing the wrong file.

This change fixes the race by using fchownat.

Scenario, as described by @slackner at
#177 :

1. Create a forward mount point with `plaintextnames` enabled
2. Mount as root user with `allow_other`
3. For testing purposes create a file `/tmp/file_owned_by_root`
   which is owned by the root user
4. As a regular user run inside of the GoCryptFS mount:

```
mkdir tempdir
mknod tempdir/file_owned_by_root p &
mv tempdir tempdir2
ln -s /tmp tempdir
```

When the steps are done fast enough and in the right order
(run in a loop!), the device file will be created in
`tempdir`, but the `lchown` will be executed by following
the symlink. As a result, the ownership of the file located
at `/tmp/file_owned_by_root` will be changed.

internal/fusefrontend/fs.go

@@ -10,6 +10,8 @@ import (
 	"syscall"
 	"time"
 
+	"golang.org/x/sys/unix"
+
 	"github.com/hanwen/go-fuse/fuse"
 	"github.com/hanwen/go-fuse/fuse/nodefs"
 	"github.com/hanwen/go-fuse/fuse/pathfs"
@@ -280,15 +282,14 @@ func (fs *FS) Mknod(path string, mode uint32, dev uint32, context *fuse.Context)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
+	dirfd, err := os.Open(filepath.Dir(cPath))
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	defer dirfd.Close()
 	// Create ".name" file to store long file name
 	cName := filepath.Base(cPath)
 	if nametransform.IsLongContent(cName) {
-		var dirfd *os.File
-		dirfd, err = os.Open(filepath.Dir(cPath))
-		if err != nil {
-			return fuse.ToStatus(err)
-		}
-		defer dirfd.Close()
 		err = fs.nameTransform.WriteLongName(dirfd, cName, path)
 		if err != nil {
 			return fuse.ToStatus(err)
@@ -300,16 +301,17 @@ func (fs *FS) Mknod(path string, mode uint32, dev uint32, context *fuse.Context)
 		}
 	} else {
 		// Create regular device node
-		err = syscall.Mknod(cPath, mode, int(dev))
+		err = syscallcompat.Mknodat(int(dirfd.Fd()), cName, mode, int(dev))
 	}
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
 	// Set owner
 	if fs.args.PreserveOwner {
-		err = os.Lchown(cPath, int(context.Owner.Uid), int(context.Owner.Gid))
+		syscall.Fchownat(int(dirfd.Fd()), cPath, int(context.Owner.Uid),
+			int(context.Owner.Gid), unix.AT_SYMLINK_NOFOLLOW)
 		if err != nil {
-			tlog.Warn.Printf("Mknod: Lchown failed: %v", err)
+			tlog.Warn.Printf("Mknod: Fchownat failed: %v", err)
 		}
 	}
 	return fuse.OK