batman-adv: Fix reference counting of vlan object for tt_local_entry

The batadv_tt_local_entry was specific to a batadv_softif_vlan and held an
implicit reference to it. But this reference was never stored in form of a
pointer in the tt_local_entry itself. Instead batadv_tt_local_remove,
batadv_tt_local_table_free and batadv_tt_local_purge_pending_clients depend
on a consistent state of bat_priv->softif_vlan_list and that
batadv_softif_vlan_get always returns the batadv_softif_vlan object which
it has a reference for. But batadv_softif_vlan_get cannot guarantee that
because it is working only with rcu_read_lock on this list. It can
therefore happen that an vid is in this list twice or that
batadv_softif_vlan_get cannot find the batadv_softif_vlan for an vid due to
some other list operations taking place at the same time.

Instead add a batadv_softif_vlan pointer directly in batadv_tt_local_entry
which will be used for the reference counter decremented on release of
batadv_tt_local_entry.

Fixes: 35df3b29

 ("batman-adv: fix TT VLAN inconsistency on VLAN re-add")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>