Staging: batman-adv: Directly prepare icmp packets in socket buffer

It is unnecessary to generate an icmp packet in an extra memory region
and than copying it to a new allocated skb.

This also resolved the problem that we do inform the user that we
couldn't send the packet because we couldn't allocate the socket buffer.

Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>