batman-adv: Fix skbuff rcsum on packet reroute

commit fc04fdb2

 upstream.

batadv_check_unicast_ttvn may redirect a packet to itself or another
originator. This involves rewriting the ttvn and the destination address in
the batadv unicast header. These field were not yet pulled (with skb rcsum
update) and thus any change to them also requires a change in the receive
checksum.

Reported-by: Matthias Schiffer <mschiffer@universe-factory.net>
Fixes: a73105b8

 ("batman-adv: improved client announcement mechanism")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>