After mounting or unmounting a filesystem, flush cache, so we don't keep
stale cache around.
RageFileDriverDirect::Remount could (and should) flush its own cache when
changing root (since the cache represents the old root). However, calling
Remount doesn't indicate that the filesystem has actually changed, so if
the root isn't actually changing (Remount("/mnt") on a filesystem already
pointing to /mnt), the driver can't be expected to flush. (Remount also
currently doesn't go through Timeout, and we need to flush on unmount,
too.) So, take the cheap way out and just flush through FILEMAN.
Glenn Maynard
4c8c5bea37