datapath: Add flow mask cache.

On every packet OVS needs to lookup flow-table with every mask
until it finds a match. The packet flow-key is first masked
with mask in the list and then the masked key is looked up in
flow-table.  Therefore number of masks can affect packet
processing performance.

Following patch adds mask index to mask cache from last
pakcet lookup in same flow.  Index of mask is stored in
this cache. This cache is searched by 5 tuple hash (skb rxhash).

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Thomas Graf <tgraf@redhat.com>