dpif-netdev: Check for PKT_RX_RSS_HASH flag.

dpif-netdev: Check for PKT_RX_RSS_HASH flag.

DPDK mbufs contain a valid RSS hash only if PKT_RX_RSS_HASH is
set in 'ol_flags'.  Otherwise the hash is garbage and doesn't
relate to the packet.

This fixes an issue with vhost, which, being a virtual NIC, doesn't
compute the hash.

Reported-by: Dongjun <dongj@dtdream.com>
Suggested-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Kevin Traynor <kevin.traynor@intel.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>