gue: fix remcsum when GRO on and CHECKSUM_PARTIAL boundary is outer UDP

In the case that GRO is turned on and the original received packet is
CHECKSUM_PARTIAL, if the outer UDP header is exactly at the last
csum-unnecessary point, which for instance could occur if the packet
comes from another Linux guest on the same Linux host, we have to do
either remcsum_adjust or set up CHECKSUM_PARTIAL again with its
csum_start properly reset considering RCO.

However, since b7fe10e5 ("gro: Fix remcsum offload to deal with frags
in GRO") that barrier in such case could be skipped if GRO turned on,
hence we pass over it and the inner L4 validation mistakenly reckons
it as a bad csum.

This patch makes remcsum_offload being reset at the same time of GRO
remcsum cleanup, so as to make it work in such case as before.

Fixes: b7fe10e5 ("gro: Fix remcsum offload to deal with frags in GRO")
Signed-off-by: Koichiro Den <den@klaipeden.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fou.c

@@ -450,6 +450,7 @@ static struct sk_buff **gue_gro_receive(struct sock *sk,
 out:
 	NAPI_GRO_CB(skb)->flush |= flush;
 	skb_gro_remcsum_cleanup(skb, &grc);
+	skb->remcsum_offload = 0;
 
 	return pp;
 }