batman-adv: Limit aggregation size to outgoing MTU

batman-adv: Limit aggregation size to outgoing MTU

If a B.A.T.M.A.N. IV aggregated OGM was prepared, it was always assumed
that 512 bytes (BATADV_MAX_AGGREGATION_BYTES) can be transmitted. But the
outgoing MTU might be too small for these 512 bytes and the aggregation
size must be adjusted in this case. Otherwise, the aggregates will cause
unnecessary packet loss.

For now, the non-aggregated packet length is not touched.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>