High: exportfs: Make fsid optional (bsc#1045161)

[krig]

The fsid export option is not needed any more. There might be some strange corner cases where it can be useful, but it definitely shouldn't be assumed to be needed by default.

The fsid=0 usage used to be important for NFSv4, but this is no longer the case. It is best not to use fsid=0 and to let exportfs/mountd create a "NFSv4 pseudo root" themselves. This is (almost) always what is wanted and is least confusing.

The fsid=N N!=0 usage was important some years ago when the device number of hard drives started to change. NFS used to use the device id, which was problematic. Setting an explicit fsid= removed the instability. However for quite some years now NFS has been using the UUID of the filesystem, rather than the device id, to identify a filesystem. So unless you have multiple filesystems with the same uuid, or are using some weird type of filesystem which doesn't have a uuid, fsid= isn't needed and is best avoided.

So exportfs_test should be changed to not require fsid= any more.

[krig]

@oalbrigt Would be grateful for some extra eyeballs on this one, in particular the cleanup_export_cache function seems to need the fsid to do cleanup. Maybe it's an acceptable tradeoff for that not to work if fsid is not explicitly configured..

[oalbrigt]

Looks good to me, but I think we should update cleanup_export_cache accordingly as well.