batman-adv: don't deal with NET_IP_ALIGN manually

Instead of dealing with NET_IP_ALIGN during allocation and
headroom reservation, it is possible to use
netdev_alloc_skb_ip_align() which transparently allocate
and reserve the correct amount of data

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>