batman-adv: Fix endianness errors for network coding

Add a htonl() in network_coding.c when reading the sequence number
from received ogm_packet, to avoid wrong byte ordering when comparing
with a host value. This bug was introduced in
3ed7ada3f0bbcd058567bc0a8f9729a73eba7db6 ("batman-adv: network coding -
detect coding nodes and remove these after timeout").

Change the type of coded_packet->coded_len from uint16 to __be16 to
avoid wrong assumptions about endianness in later uses. Introduced in
c3289f3650d34b60296000a629c99f2488f7c3dd ("batman-adv: network coding -
code and transmit packets if possible").

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Martin Hundebøll <martin@hundeboll.net>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>