batman-adv: generalize batman-adv icmp packet handling

Instead of handling icmp packets only up to length of icmp_packet_rr,
the code should handle any icmp length size. Therefore the length
truncating is moved to when the packet is actually sent to userspace
(this does not support lengths longer than icmp_packet_rr yet). Longer
packets are forwarded without truncating.

This patch also cleans up some parts where the icmp header struct could
be used instead of other icmp_packet(_rr) structs to make the code more
readable.

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>
diff --git a/net/batman-adv/packet.h b/net/batman-adv/packet.h
index 843b96a..207459b 100644
--- a/net/batman-adv/packet.h
+++ b/net/batman-adv/packet.h
@@ -239,6 +239,8 @@
 	uint8_t  rr[BATADV_RR_LEN][ETH_ALEN];
 };
 
+#define BATADV_ICMP_MAX_PACKET_SIZE	sizeof(struct batadv_icmp_packet_rr)
+
 /* All packet headers in front of an ethernet header have to be completely
  * divisible by 2 but not by 4 to make the payload after the ethernet
  * header again 4 bytes boundary aligned.