fs: support relatime correctly for CephFS
As of now, CephFS does not seem to handle atime.
The relatime mount option (since Linux 2.6.30) is meant to not only adapt the atime to mtime/ctime, but will also bump the atime when an access happens after 24 hours.
There is one major use case for this we are interested in:
Find old data not read anymore. This use case probably applies to most long-lived clusters with actual users who tend not to clean up, but keep data forever even if not needed anymore.
From a more positive viewpoint, it can also help users themselves to clean up.
Bumping the atime at maximum once per day (as is done for other FS) seems reasonable for performance reasons and does not destroy this use case.