batman-adv: privatize forw_packet skb assignment

An skb is assigned to a forw_packet only once, shortly after the
forw_packet allocation.

With this patch the assignment is moved into the this allocation
function.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>