batman-adv: use kmem_cache for translation table

The translation table (global, local) is usually the part of batman-adv
which has the most dynamical allocated objects. Most of them
(tt_local_entry, tt_global_entry, tt_orig_list_entry, tt_change_node,
tt_req_node, tt_roam_node) are equally sized. So it makes sense to have
them allocated from a kmem_cache for each type.

This approach allowed a small wireless router (TP-Link TL-841NDv8; SLUB
allocator) to store 34% more translation table entries compared to the
current implementation.

[1] https://open-mesh.org/projects/batman-adv/wiki/Kmalloc-kmem-cache-tests

Reported-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/translation-table.h

@@ -59,4 +59,7 @@ bool batadv_tt_add_temporary_global_entry(struct batadv_priv *bat_priv,
 bool batadv_tt_global_is_isolated(struct batadv_priv *bat_priv,
 				  const u8 *addr, unsigned short vid);
 
+int batadv_tt_cache_init(void);
+void batadv_tt_cache_destroy(void);
+
 #endif /* _NET_BATMAN_ADV_TRANSLATION_TABLE_H_ */