batman-adv: set TT_CLIENT_NEW flag before invoking hash_add()

In case of a new tt_local_entry, the TT_CLIENT_NEW flag has to be set before
adding such entry to the hash table. Otherwise, it opens a race condition
in which the entry can be found but the flag has not been set.

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