Add calls to add and remove flows to the predt_table. This very simply
just allocates and add a new pretun entry if detected as such, and
removes it when encountered on a delete flow.

Compatibility for older firmware is kept in place through the
DECAP_V2 feature bit.
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>