batman-adv: clear control block of received socket buffers

Since other network components (and some drivers) uses the control block
provided in skb's, the network coding feature might wrongly assume that
an SKB has been decoded, and thus not try to code it with another packet
again. This happens for instance when batman-adv is running on a bridge device.

Fix this by clearing the control block for every received SKB.

Introduced by 3c12de9a
("batman-adv: network coding - code and transmit packets if possible")
Signed-off-by: Martin Hundebøll <martin@hundeboll.net>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>

Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>