Staging: batman-adv: Provide full headers and packets as linear skb

We must ensure that all interesting data is linear and not paged out to
access all information in a header or a full batman-adv related packet.
Otherwise we may drop packets which have non-linear headers but which
hold valid data.

This doesn't affect non-linear skbs which have all headers in a linear
head unless we must process the whole packet like in ogms or vis
packets.

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