net/mlx5e: Create route entry infrastructure

Implement dedicated route entry infrastructure to be used in following
patch by route update event. Both encap (indirectly through their
corresponding encap entries) and decap (directly) flows are attached to
routing entry. Since route update also requires updating encap (route
device MAC address is a source MAC address of tunnel encapsulation), same
encap_tbl_lock mutex is used for synchronization.

The new infrastructure looks similar to existing infrastructures for shared
encap, mod_hdr and hairpin entries:

- Per-eswitch hash table is used for quick entry lookup.

- Flows are attached to per-entry linked list and hold reference to entry
  during their lifetime.

- Atomic reference counting and rcu mechanisms are used as synchronization
  primitives for concurrent access.

The infrastructure also enables connection tracking on stacked devices
topology by attaching CT chain 0 flow on tunneling dev to decap route
entry.

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Dmytro Linkin <dlinkin@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>