Don't wait to flush writes, as we don't care about consistency on a
crash:
> This option allows the NFS server to violate the NFS protocol and
> reply to requests before any changes made by that request have been
> committed to stable storage (e.g. disc drive).
>
> Using this option usually improves performance, but at the cost that
> an unclean server restart (i.e. a crash) can cause data to be lost or
> corrupted.
Reviewed-by: Aleix Boné <abonerib@bsc.es>
