Staging: batman-adv: Only clone skb data for multiple broadcasts

batman-adv tries to resend broadcasts on all interfaces up to three
times. For each round and each interface it must provide a skb which
gets consumed by the sending function.

It is unnecessary to copy the data of each broadcast because the actual
data is either not shared or already copied by add_bcast_packet_to_list.
So it is enough to just copy the skb control data

Reported-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>