mlxsw: spectrum_router: Make route creation and destruction symmetric

Host routes that perform decapsulation of IP in IP tunnels have a
special adjacency entry linked to them. This entry stores information
such as the expected underlay source IP. When the route is deleted this
entry needs to be freed.

The allocation of the adjacency entry happens in
mlxsw_sp_fib4_entry_type_set(), but it is freed in
mlxsw_sp_fib4_node_entry_unlink().

Create a new function - mlxsw_sp_fib4_entry_type_unset() - and free the
adjacency entry there.

This will allow us to consolidate mlxsw_sp_fib{4,6}_node_entry_unlink()
in the next patch.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>