batman-adv: bat_socket_read missing checks

commit c00b6856fc642b234895cfabd15b289e76726430 upstream.

Writing a icmp_packet_rr and then reading icmp_packet can lead to kernel
memory corruption, if __user *buf is just below TASK_SIZE.

Signed-off-by: Paul Kot <pawlkt@gmail.com>
[sven@narfation.org: made it checkpatch clean]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>