dpif: Add support for OVS_ACTION_ATTR_CT_CLEAR

dpif: Add support for OVS_ACTION_ATTR_CT_CLEAR

This supports using the ct_clear action in the kernel datapath. To
preserve compatibility with current ct_clear behavior on old kernels, we
only pass this action down to the datapath if a probe reveals the
datapath actually supports it.

Signed-off-by: Eric Garver <e@erig.me>
Acked-by: William Tu <u9012063@gmail.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Justin Pettit <jpettit@ovn.org>