mount.nfs: Don't allow the user to specify addr= or clientaddr=.

The current mount.nfs implementation doesn't allow users to specify their
own addr= or clientaddr= option.  The new string-based interface does
allow this, even though nfs(5) does not document 'addr=' and specifically
forbids adding 'clientaddr='.

Make the addition of either option by the user a permanent error.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>