batman-adv: prefer kfree_rcu() over call_rcu() with free-only callbacks

Drop 'batadv_tt_local_entry_free_rcu()', 'batadv_tt_global_entry_free_rcu()'
and 'batadv_tt_orig_list_entry_free_rcu()' in favor of 'kfree_rcu()' in
'batadv_tt_local_entry_release()', 'batadv_tt_global_entry_release()' and
'batadv_tt_orig_list_entry_release()', respectively.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>