batman-adv: check return value of pskb_trim_rcsum()

Reported-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Acked-by: Martin Hundebøll <martin@hundeboll.net>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>