Add ability for the datapath to match IP address in ARPs

The ability to match the IP addresses in ARP packets allows for fine-grained
control of ARP processing.  Some forthcoming changes to allow in-band
control to operate over L3 requires this support if we don't want to
allow overly broad rules regarding ARPs to always be white-listed.
Unfortunately, OpenFlow does not support this sort of processing yet, so
we must treat OpenFlow ARP rules as having wildcarded those L3 fields.

include/openvswitch/datapath-protocol.h

@@ -162,7 +162,8 @@ struct odp_flow_key {
     __be16 tp_dst;               /* TCP/UDP destination port. */
     __u8   dl_src[ETH_ALEN];     /* Ethernet source address. */
     __u8   dl_dst[ETH_ALEN];     /* Ethernet destination address. */
-    __u8   nw_proto;             /* IP protocol. */
+    __u8   nw_proto;             /* IP protocol or lower 8 bits of 
+                                    ARP opcode. */
     __u8   reserved;             /* Pad to 64 bits. */
 };