datapath: Drop support for kernel older than 3.10

datapath: Drop support for kernel older than 3.10

Currently OVS out of tree datapath supports a large number of kernel
versions. From 2.6.32 to 4.3 and various distribution-specific
kernels. But at this point major features are only available on more
recent kernels.  For example, stateful services are only available
starting in kernel 3.10 and STT is available on starting with 3.5.

Since these features are becoming essential to many OVS deployments,
and the effort of maintaining the backports is high. We have decided
to drop support for older kernel. Following patch drops supports
for kernel older than 3.10.

Signed-off-by: Pravin B Shelar <pshelar@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>