net: gro: make sure skb->cb[] initial content has not to be zero

net: gro: make sure skb->cb[] initial content has not to be zero

Starting from linux-3.13, GRO attempts to build full size skbs.

Problem is the commit assumed one particular field in skb->cb[]
was clean, but it is not the case on some stacked devices.

Timo reported a crash in case traffic is decrypted before
reaching a GRE device.

Fix this by initializing NAPI_GRO_CB(skb)->last at the right place,
this also removes one conditional.

Thanks a lot to Timo for providing full reports and bisecting this.

Fixes: 8a29111c7ca6 ("net: gro: allow to build full sized skb")
Bisected-by: Timo Teras <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Tested-by: Timo Teräs <[email protected]>
Signed-off-by: David S. Miller <[email protected]>