batman-adv: fix potential kernel paging errors for unicast transmissions

There are several functions which might reallocate skb data. Currently
some places keep reusing their old ethhdr pointer regardless of whether
they became invalid after such a reallocation or not. This potentially
leads to kernel paging errors.

This patch fixes these by refetching the ethdr pointer after the
potential reallocations.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>