batman-adv: select an internet gateway if none was chosen

This is a regression introduced by: 2265c141086474bbae55a5bb3afa1ebb78ccaa7c
("batman-adv: gateway election code refactoring")

Reported-by: Nicolás Echániz <nicoechaniz@codigosur.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Acked-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: David S. Miller <davem@davemloft.net>