nfsd: Add a super simple flex file server

Have a simple flex file server where the mds (NFSv4.1 or NFSv4.2)
is also the ds (NFSv3). I.e., the metadata and the data file are
the exact same file.

This will allow testing of the flex file client.

Simply add the "pnfs" export option to your export
in /etc/exports and mount from a client that supports
flex files.

Signed-off-by: Tom Haynes <loghyr@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>