batman-adv: Linearize fragment packets before merge

We access the data inside the skbs of two fragments directly using memmove
during the merge. The data of the skb could span over multiple skb pages. An
direct access without knowledge about the pages would lead to an invalid memory
access.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
[lindner_marek@yahoo.de: Move return from function to the end]
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>