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: f1bd7d659ef0 ("xfrm: Add encapsulation header offsets while SKB is not encrypted")
Reported-by: Vicente De Luca <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

net/xfrm/xfrm_output.c

···
 		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;
···
 	int err;
 
 	secpath_reset(skb);
-	skb->encapsulation = 0;
 
 	if (xfrm_dev_offload_ok(skb, x)) {
 		struct sec_path *sp;