ofproto-dpif: Allow IPv6 ND Extensions only if supported

ofproto-dpif: Allow IPv6 ND Extensions only if supported

The IPv6 ND Extensions is only implemented in userspace datapath,
but nothing prevents that to be used with other datapaths.

This patch probes the datapath and only allows if the support
is available.

Fixes: 9b2b84973 ("Support for match & set ICMPv6 reserved and options type fields")
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>