vfs: fix modtime set if --vfs-cache-mode writes/full and no write

When using --vfs-cache-mode writes or full if a file was opened for
write intent, the modtime was set and the file was closed without
being modified the modtime would never be written back to storage.

The sequence of events

- app opens file with write intent
- app does set modtime
- rclone sets the modtime on the cache file, but not the remote file
  because it is open for write and can't be set yet
- app closes the file without changing it
- rclone doesn't upload the file because the file wasn't changed so
  the modtime doesn't get updated

This fixes the problem by making sure any unapplied modtime changes
are applied even if the file is not modified when being closed.

Fixes #4795

vfs/file.go

@@ -424,6 +424,13 @@ func (f *File) _applyPendingModTime() error {
 	return nil
 }
 
+// Apply a pending mod time
+func (f *File) applyPendingModTime() error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	return f._applyPendingModTime()
+}
+
 // _writingInProgress returns true of there are any open writers
 // Call with read lock held
 func (f *File) _writingInProgress() bool {