batman-adv: Treat NET_XMIT_CN as transmit successfully

The tc could return NET_XMIT_CN as one congestion notification, but
it does not mean the packet is lost. Other modules like ipvlan,
macvlan, and others treat NET_XMIT_CN as success too.

So batman-adv should handle NET_XMIT_CN also as NET_XMIT_SUCCESS.

Signed-off-by: Gao Feng <gfree.wind@gmail.com>
[sven@narfation.org: Moved NET_XMIT_CN handling to batadv_send_skb_packet]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>