fuse: fix DirectMount mixups

Two variable were defined but never used in what looks like
a copy-paste error or something.

Fix that, which also makes the tests pass.

Change-Id: Iebf48583874977c518d888d9a80bc1f02e2a7f86

fuse/api.go

@@ -256,8 +256,11 @@ type MountOptions struct {
 	// DirectMountStrict wins.
 	DirectMountStrict bool
 
-	// Options passed to syscall.Mount, the default value used by fusermount
-	// is syscall.MS_NOSUID|syscall.MS_NODEV
+	// DirectMountFlags are the mountflags passed to syscall.Mount. If zero, the
+	// default value used by fusermount are used: syscall.MS_NOSUID|syscall.MS_NODEV.
+	//
+	// If you actually *want* zero flags, pass syscall.MS_MGC_VAL, which is ignored
+	// by the kernel. See `man 2 mount` for details about MS_MGC_VAL.
 	DirectMountFlags uintptr
 
 	// EnableAcls enables kernel ACL support.

fuse/mount_linux.go

@@ -41,8 +41,10 @@ func mountDirect(mountPoint string, opts *MountOptions, ready chan<- error) (fd
 		source = opts.Name
 	}
 
-	var flags uintptr
-	flags |= syscall.MS_NOSUID | syscall.MS_NODEV
+	var flags uintptr = syscall.MS_NOSUID | syscall.MS_NODEV
+	if opts.DirectMountFlags != 0 {
+		flags = opts.DirectMountFlags
+	}
 
 	// some values we need to pass to mount, but override possible since opts.Options comes after
 	var r = []string{
@@ -59,9 +61,9 @@ func mountDirect(mountPoint string, opts *MountOptions, ready chan<- error) (fd
 
 	if opts.Debug {
 		log.Printf("mountDirect: calling syscall.Mount(%q, %q, %q, %#x, %q)",
-			opts.FsName, mountPoint, "fuse."+opts.Name, opts.DirectMountFlags, strings.Join(r, ","))
+			source, mountPoint, "fuse."+opts.Name, flags, strings.Join(r, ","))
 	}
-	err = syscall.Mount(opts.FsName, mountPoint, "fuse."+opts.Name, opts.DirectMountFlags, strings.Join(r, ","))
+	err = syscall.Mount(source, mountPoint, "fuse."+opts.Name, flags, strings.Join(r, ","))
 	if err != nil {
 		syscall.Close(fd)
 		return