packet-dpif: Add dpif_packet_{get, set}_hash()

packet-dpif: Add dpif_packet_{get, set}_hash()

These function are used to stored the packet hash. 'netdev-dpdk'
automatically set this value to the RSS hash returned by the
NIC. Other 'netdev's set it to 0 (which is an invalid hash
value), so that callers can compute the hash on their own.

If DPDK support is enabled, struct dpif_packet's member
'dp_hash' is removed and 'pkt.hash.rss' from DPDK mbuf is used

This commit also configure DPDK devices to compute RSS hash
for UDP and IPv6 packets

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>