xfrm: Fix GSO for IPsec with GRE tunnel.

We reset the encapsulation field of the skb too early
in xfrm_output. As a result, the GRE GSO handler does
not segment the packets. This leads to a performance
drop down. We fix this by resetting the encapsulation
field right before we do the transformation, when
the inner headers become invalid.

Fixes: f1bd7d659e ("xfrm: Add encapsulation header offsets while SKB is not encrypted")
Reported-by: Vicente De Luca <vdeluca@zendesk.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_output.c

@@ -105,6 +105,9 @@ static int xfrm_output_one(struct sk_buff *skb, int err)
 		if (xfrm_offload(skb)) {
 			x->type_offload->encap(x, skb);
 		} else {
+			/* Inner headers are invalid now. */
+			skb->encapsulation = 0;
+
 			err = x->type->output(x, skb);
 			if (err == -EINPROGRESS)
 				goto out;
@@ -208,7 +211,6 @@ int xfrm_output(struct sock *sk, struct sk_buff *skb)
 	int err;
 
 	secpath_reset(skb);
-	skb->encapsulation = 0;
 
 	if (xfrm_dev_offload_ok(skb, x)) {
 		struct sec_path *sp;