nfsd: clean up fh_auth usage

Use fh_fsid when reffering to the fsid part of the filehandle.  The
variable length auth field envisioned in nfsfh wasn't ever implemented.
Also clean up some lose ends around this and document the file handle
format better.

Btw, why do we even export nfsfh.h to userspace?  The file handle very
much is kernel private, and nothing in nfs-utils include the header
either.

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