NFSv4: Turn off open-by-filehandle and NFS re-export for NFSv4.0

The NFSv4.0 protocol only supports open() by name. It cannot therefore
be used with open_by_handle() and friends, nor can it be re-exported by
knfsd.

Reported-by: Chuck Lever III <chuck.lever@oracle.com>
Fixes: 20fa1902 ("nfs: add export operations")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>