batman-adv: Consider fragmentation for needed_headroom

If a batman-adv packets has to be fragmented, then the original batman-adv
packet header is not stripped away. Instead, only a new header is added in
front of the packet after it was split.

This size must be considered to avoid cost intensive reallocations during
the transmission through the various device layers.

Fixes: 7bca68c7 ("batman-adv: Add lower layer needed_(head|tail)room to own ones")
Reported-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>