mount: fix --devname and fusermount: unknown option 'fsname' when mounting via rc

In this commit

f4c40bf79d mount: add --devname to set the device name sent to FUSE for mount display

The --devname parameter was added. However it was soon noticed that
attempting to mount via the rc gave this error:

    mount helper error: fusermount: unknown option 'fsname'
    mount FAILED: fusermount: exit status 1

This was because the DeviceName (and VolumeName) parameter was never
being initialised when the mount was called via the rc.

The fix for this was to refactor the rc interface so it called the
same Mount method as the command line mount which initialised the
DeviceName and VolumeName parameters properly.

This also fixes the cmd/mount tests which were breaking in the same
way but since they aren't normally run on the CI we didn't notice.

Fixes #6044

cmd/mountlib/mount.go

@@ -78,6 +78,17 @@ type MountPoint struct {
 	ErrChan    <-chan error
 }
 
+// NewMountPoint makes a new mounting structure
+func NewMountPoint(mount MountFn, mountPoint string, f fs.Fs, mountOpt *Options, vfsOpt *vfscommon.Options) *MountPoint {
+	return &MountPoint{
+		MountFn:    mount,
+		MountPoint: mountPoint,
+		Fs:         f,
+		MountOpt:   *mountOpt,
+		VFSOpt:     *vfsOpt,
+	}
+}
+
 // Global constants
 const (
 	MaxLeafSize = 1024 // don't pass file names longer than this
@@ -167,14 +178,7 @@ func NewMountCommand(commandName string, hidden bool, mount MountFn) *cobra.Comm
 				defer cmd.StartStats()()
 			}
 
-			mnt := &MountPoint{
-				MountFn:    mount,
-				MountPoint: args[1],
-				Fs:         cmd.NewFsDir(args),
-				MountOpt:   Opt,
-				VFSOpt:     vfsflags.Opt,
-			}
-
+			mnt := NewMountPoint(mount, args[1], cmd.NewFsDir(args), &Opt, &vfsflags.Opt)
 			daemon, err := mnt.Mount()
 
 			// Wait for foreground mount, if any...
@@ -253,6 +257,7 @@ func (m *MountPoint) Mount() (daemon *os.Process, err error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to mount FUSE fs: %w", err)
 	}
+	m.MountedOn = time.Now()
 	return nil, nil
 }