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>