datapath: Provide compatibility for kernels up to 3.17

Port datapath to work with kernrels up to 3.17 and use 3.16.2 as
the new kernel for CI testing.

Tested with 3.14, 3.16.2, and net-next (3.17).

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Co-authored-by: Madhu Challa <challa@noironetworks.com>