vfs: Ignore Truncate if called with no readers and already the correct size

This fixes FreeBSD which seems to call SetAttr with a size even on
read only files.

This is probably a bug in the FreeBSD FUSE implementation as it
happens with mount and cmount.

See: https://forum.rclone.org/t/freebsd-question/8662/12

vfs/file.go

@@ -601,6 +601,12 @@ func (f *File) Truncate(size int64) (err error) {
 		}
 		return err
 	}
+
+	// If no writers, and size is already correct then all done
+	if f.o.Size() == size {
+		return nil
+	}
+
 	fs.Debugf(f.o, "Truncating file")
 
 	// Otherwise if no writers then truncate the file by opening