batman-adv: ensure bcast is writable before modifying TTL

batman-adv: ensure bcast is writable before modifying TTL

Before batman-adv is allowed to write to an skb, it either has to have its
own copy of the skb or used skb_cow() to ensure that the data part is not
shared.

The old implementation used a shared queue and created copies before
attempting to write to it. But with the new implementation, the broadcast
packet is already modified when it gets received. Potentially writing to
shared buffers in this process.

Adding a skb_cow() right before this operation avoids this and can at the
same time prepare it for the modifications required to rebroadcast the
packet.

Cc: stable@kernel.org
Fixes: 3f69339068f9 ("batman-adv: bcast: queue per interface, if needed")
Signed-off-by: Sven Eckelmann <sven@narfation.org>