NFS proxy-only mode

From Linux NFS

It could be useful to have a mode where an NFS server is dedicated to reexporting all the exports from *one* other NFS server. It would have no other exports whatsoever.

This would allow the re-export server to support crossmount-like behavior, skip adding its own filesystem identifier to each filehandle (fixing problems with filehandle length limits), and avoid the need for manual assignment of filesystem identifiers with the fsid= option.

Possible implementation (needs more details):

- provide interface to turn on this mode and identify the original server. This could be a new call to be made before starting nfsd, or it could use the export upcall to mountd somehow.

- add a new export operation which accepts a filehandle and returns a superblock.