batman-adv: don't use call_rcu if not needed

batadv_tt_global_entry_free_ref uses call_rcu to schedule a
function which will only free the global entry itself.

For this reason call_rcu is useless and kfree_rcu can be
used to simplify the code.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>