Staging: batman-adv: Clone shared bat packets before modifying them

"tcpdump" and "batctl td" will receive packets with a wrong sequence
number on systems with a different endianess than network byte order.
This happens due to the reordering of bytes in the function which
handles aggregated bat packets. The function which receives the bat
packets must ensure that these buffers aren't shared with anything else
before that function tries to write into it. Otherwise it has to copy
the buffers so it is save again to change them.

Reported-by: Kevin Steen <batman@kevinsteen.net>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>