cifs: mark CONFIG_CIFS_NFSD_EXPORT as BROKEN

This does not work properly with CIFS as current servers do not
enable support for the FILE_OPEN_BY_FILE_ID on SMB NTCreateX
and not all NFS clients handle ESTALE.

For now, it just plain doesn't work. Mark it BROKEN to discourage
distros from enabling it.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>