datapath: Add this_cpu_{read, inc, dec} APIs for backward compatibility

The upstream modules uses this_cpu_xxx APIs. Add those functions for
older kernel (<3.0.0) that does not provide them.

VMware-BZ: #1319082

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