i40e: Fix flow for IPv6 next header (extension header)

When a packet contains an IPv6 header with next header which is
an extension header and not a protocol one, the kernel function
skb_transport_header called with such sk_buff will return a
pointer to the extension header and not to the TCP one.

The above explained call caused a problem with packet processing
for skb with encapsulation for tunnel with I40E_TX_CTX_EXT_IP_IPV6.
The extension header was not skipped at all.

The ipv6_skip_exthdr function does check if next header of the IPV6
header is an extension header and doesn't modify the l4_proto pointer
if it points to a protocol header value so its safe to omit the
comparison of exthdr and l4.hdr pointers. The ipv6_skip_exthdr can
return value -1. This means that the skipping process failed
and there is something wrong with the packet so it will be dropped.

Fixes: a3fd9d88 ("i40e/i40evf: Handle IPv6 extension headers in checksum offload")
Signed-off-by: NSlawomir Laba <slawomirx.laba@intel.com>
Signed-off-by: NPrzemyslaw Patynowski <przemyslawx.patynowski@intel.com>
Reviewed-by: NAleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: NTony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: NTony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/i40e/i40e_txrx.c

@@ -3113,13 +3113,16 @@ static int i40e_tx_enable_csum(struct sk_buff *skb, u32 *tx_flags,
 
 			l4_proto = ip.v4->protocol;
 		} else if (*tx_flags & I40E_TX_FLAGS_IPV6) {
+			int ret;
+
 			tunnel |= I40E_TX_CTX_EXT_IP_IPV6;
 
 			exthdr = ip.hdr + sizeof(*ip.v6);
 			l4_proto = ip.v6->nexthdr;
-			if (l4.hdr != exthdr)
-				ipv6_skip_exthdr(skb, exthdr - skb->data,
-						 &l4_proto, &frag_off);
+			ret = ipv6_skip_exthdr(skb, exthdr - skb->data,
+					       &l4_proto, &frag_off);
+			if (ret < 0)
+				return -1;
 		}
 
 		/* define outer transport */