datapath: Add support for kernels 3.13

datapath: Add support for kernels 3.13

Add support for building the in-tree kernel datapath for
Linux kernels up to 3.13. There were some changes in the
netlink area which required adding new compatibility code
for this layer. Also, some new per-cpu stats initialization
code was added.

Based on patch from Kyle Mestery.

Signed-off-by: Kyle Mestery <mestery@noironetworks.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Kyle Mestery <mestery@noironetworks.com>