batman-adv: Directly check read of icmp packet in copy_from_user

The access_ok read check can be directly done in copy_from_user since a failure
of access_ok is handled the same way as an error in __copy_from_user.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>