ice: Implement flow for IPv6 next header (extension header)

This patch is based on a similar change to i40e by Slawomir Laba:
"i40e: Implement 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 ICE_TX_CTX_EIPT_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: a4e82a81 ("ice: Add support for tunnel offloads")
Signed-off-by: Nick Nunley <nicholas.d.nunley@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/ice/ice_txrx.c

@@ -1923,12 +1923,15 @@ int ice_tx_csum(struct ice_tx_buf *first, struct ice_tx_offload_params *off)
 				  ICE_TX_CTX_EIPT_IPV4_NO_CSUM;
 			l4_proto = ip.v4->protocol;
 		} else if (first->tx_flags & ICE_TX_FLAGS_IPV6) {
+			int ret;
+
 			tunnel |= ICE_TX_CTX_EIPT_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 */