batman-adv: substitute tt_poss_change with a per-tt_entry flag

tt_poss_change is a node-wide flag which tells whether the node is in a roaming
state (a client recently moved to/away from it) in order to let it apply special
re-routing rules. However this flag does not give a clear idea of the current
state because it is not possible to understand *which client* is actually
involved in the roaming. For this reason a better approach has been chosen:
instead of using a node-wide variable, the roaming state is now given by a
per-tt_entry ROAM flag which, in case of packet coming through the node, tells
the node whether the real destination is in roaming state or not.

With this flag change, batadv_check_unicast_ttvn() has also been rearranged in
order to better fit the new re-routing logic and to be much more readable.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>

net/batman-adv/soft-interface.c

@@ -505,7 +505,6 @@ struct net_device *batadv_softif_create(const char *name)
 #endif
 	bat_priv->tt.last_changeset = NULL;
 	bat_priv->tt.last_changeset_len = 0;
-	bat_priv->tt.poss_change = false;
 
 	bat_priv->primary_if = NULL;
 	bat_priv->num_ifaces = 0;