nicira-ext: Support matching IPv6 Neighbor Discovery messages.

nicira-ext: Support matching IPv6 Neighbor Discovery messages.

IPv6 uses Neighbor Discovery messages in a similar manner to how IPv4
uses ARP.  This commit adds support for matching deeper into the
payloads of Neighbor Solicitation (NS) and Neighbor Advertisement (NA)
messages.  Currently, the matching fields include:

    - NS and NA Target (nd_target)
    - NS Source Link Layer Address (nd_sll)
    - NA Target Link Layer Address (nd_tll)

When defining IPv6 Neighbor Discovery rules, the Nicira Extensible Match
(NXM) extension to OVS must be used.

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>