datapath: Simplify flow mask cache delete.

Currently on mask delete OVS moves last mask to the deleted
place to keep flow cache consistent and compact for per cpu
cache. But that generate duplicate entries in mask cache array
which results in multiple flow lookups in case we miss flow
cache.
Following patch simply sets NULL for deleted entry.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>