batman-adv: allow bla traffic only after first worker period

When adding a backbone gateway for the first time, it might not yet
be known in the backbone, and therefore we should not forward
broadcasts yet. This behaviour is the same as when sending a request
to another backbone gw because of a CRC mismatch. The backbone gw
will operate normal after the next periodic bla work.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>