batman-adv: Fix mem leak in the batadv_tt_local_event() function

Memory is allocated for 'tt_change_node' with kmalloc().
'tt_change_node' may go out of scope really being used for anything
(except have a few members initialized) if we hit the 'del:' label.
This patch makes sure we free the memory in that case.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: David S. Miller <davem@davemloft.net>