nfp: flower: add infrastructure for pre_tun rework

The previous implementation of using a pre_tun_table for decap has
some limitations, causing flows to end up unoffloaded when in fact
we are able to offload them. This is because the pre_tun_table does
not have enough matching resolution. The next step is to instead make
use of the neighbour table which already exists for the encap direction.
This patch prepares for this by:

- Moving nfp_tun_neigh/_v6 to main.h.
- Creating two new "wrapping" structures, one to keep track of neighbour
  entries (previously they were send-and-forget), and another to keep
  track of pre_tun flows.
- Create a new list in nfp_flower_priv to keep track of pre_tunnel flows
- Create a new table in nfp_flower_priv to keep track of next neighbour
  entries
- Initialising and destroying these new list/tables
- Extending nfp_fl_payload->pre_tun_rule to save more information for
  future use.

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>