selftests: openvswitch: Skip drop testing on older kernels

Kernels that don't have support for openvswitch drop reasons also
won't have the drop counter reasons, so we should skip the test
completely.  It previously wasn't possible to build a test case
for this without polluting the datapath, so we introduce a mechanism
to clear all the flows from a datapath allowing us to test for
explicit drop actions, and then clear the flows to build the
original test case.

Fixes: 42420291 ("selftests: openvswitch: add explicit drop testcase")
Signed-off-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>