nicira-ext: Support matching ARP source and target hardware addresses.

nicira-ext: Support matching ARP source and target hardware addresses.

OpenFlow 1.0 doesn't allow matching on the ARP source and target
hardware address.  This has caused us to introduce hacks such as the
Drop Spoofed ARP action.  Now that we have extensible match, we can
match on more fields within ARP:

    - Source Hardware Address (arp_sha)
    - Target Hardware Address (arp_tha)

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