batman-adv: Keep fragments equally sized

The batman-adv fragmentation packets have the design problem that they
cannot be refragmented and cannot handle padding by the underlying link.
The latter often leads to problems when networks are incorrectly configured
and don't use a common MTU.

The sender could for example fragment a 1271 byte frame (plus external
ethernet header (14) and batadv unicast header (10)) to fit in a 1280 bytes
large MTU of the underlying link (max. 1294 byte frames). This would create
a 1294 bytes large frame (fragment 2) and a 55 bytes large frame
(fragment 1). The extra 54 bytes are the fragment header (20) added to each
fragment and the external ethernet header (14) for the second fragment.

Let us assume that the next hop is then not able to transport 1294 bytes to
its next hop. The 1294 byte large frame will be dropped but the 55 bytes
large fragment will still be forwarded to its destination.

Or let us assume that the underlying hardware requires that each frame has
a minimum size (e.g. 60 bytes). Then it will pad the 55 bytes frame to 60
bytes. The receiver of the 60 bytes frame will no longer be able to
correctly assemble the two frames together because it is not aware that 5
bytes of the 60 bytes frame are padding and don't belong to the reassembled
frame.

This can partly be avoided by splitting frames more equally. In this
example, the 675 and 674 bytes large fragment frames could both potentially
reach its destination without being too large or too small.

Reported-by: Martin Weinelt <martin@darmstadt.freifunk.net>
Fixes: ee75ed88

 ("batman-adv: Fragment and send skbs larger than mtu")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>