nfsd: export proper maximum file size to the client

I noticed that we export a way to high value for the maxfilesize
attribute when debugging a client issue.  The issue didn't turn
out to be related to it, but I think we should export it, so that
clients can limit what write sizes they accept before hitting
the server.

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