ip6_gre: Revert "ip6_gre: add validation for csum_start"

This reverts commit 9cf448c200ba9935baa94e7a0964598ce947db9d.

This commit was added for equivalence with a similar fix to ip_gre.
That fix proved to have a bug. Upon closer inspection, ip6_gre is not
susceptible to the original bug.

So revert the unnecessary extra check.

In short, ipgre_xmit calls skb_pull to remove ipv4 headers previously
inserted by dev_hard_header. ip6gre_tunnel_xmit does not.

Link: https://lore.kernel.org/netdev/CA+FuTSe+vJgTVLc9SojGuN-f9YQ+xWLPKE_S4f=f+w+_P2hgUg@mail.gmail.com/#t
Fixes: 9cf448c200ba ("ip6_gre: add validation for csum_start")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 7baf41d..3ad201d 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -629,8 +629,6 @@
 static int gre_handle_offloads(struct sk_buff *skb, bool csum)
-	if (csum && skb_checksum_start(skb) < skb->data)
-		return -EINVAL;
 	return iptunnel_handle_offloads(skb,
 					csum ? SKB_GSO_GRE_CSUM : SKB_GSO_GRE);