Add Nicira extension to OpenFlow for dropping spoofed ARP packets.

Add Nicira extension to OpenFlow for dropping spoofed ARP packets.

"ARP spoofing" is when a host claims an incorrect association between an
IP address and a MAC address for deceptive purposes.  OpenFlow by itself
can prevent a host from sending out ARP replies from an incorrect MAC
address in the Ethernet L2 header, but it cannot control the MAC addresses
inside the ARP L3 packet.  This commit adds a new action that can be used
to drop these spoofed packets.

CC: Paul Ingram <paul@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>