net: dsa: fix flow dissection on Tx path

Commit 43e66528 ("net-next: dsa: fix flow dissection") added an
ability to override protocol and network offset during flow dissection
for DSA-enabled devices (i.e. controllers shipped as switch CPU ports)
in order to fix skb hashing for RPS on Rx path.

However, skb_hash() and added part of code can be invoked not only on
Rx, but also on Tx path if we have a multi-queued device and:
 - kernel is running on UP system or
 - XPS is not configured.

The call stack in this two cases will be like: dev_queue_xmit() ->
__dev_queue_xmit() -> netdev_core_pick_tx() -> netdev_pick_tx() ->
skb_tx_hash() -> skb_get_hash().

The problem is that skbs queued for Tx have both network offset and
correct protocol already set up even after inserting a CPU tag by DSA
tagger, so calling tag_ops->flow_dissect() on this path actually only
breaks flow dissection and hashing.

This can be observed by adding debug prints just before and right after
tag_ops->flow_dissect() call to the related block of code:

Before the patch:

Rx path (RPS):

[   19.240001] Rx: proto: 0x00f8, nhoff: 0	/* ETH_P_XDSA */
[   19.244271] tag_ops->flow_dissect()
[   19.247811] Rx: proto: 0x0800, nhoff: 8	/* ETH_P_IP */

[   19.215435] Rx: proto: 0x00f8, nhoff: 0	/* ETH_P_XDSA */
[   19.219746] tag_ops->flow_dissect()
[   19.223241] Rx: proto: 0x0806, nhoff: 8	/* ETH_P_ARP */

[   18.654057] Rx: proto: 0x00f8, nhoff: 0	/* ETH_P_XDSA */
[   18.658332] tag_ops->flow_dissect()
[   18.661826] Rx: proto: 0x8100, nhoff: 8	/* ETH_P_8021Q */

Tx path (UP system):

[   18.759560] Tx: proto: 0x0800, nhoff: 26	/* ETH_P_IP */
[   18.763933] tag_ops->flow_dissect()
[   18.767485] Tx: proto: 0x920b, nhoff: 34	/* junk */

[   22.800020] Tx: proto: 0x0806, nhoff: 26	/* ETH_P_ARP */
[   22.804392] tag_ops->flow_dissect()
[   22.807921] Tx: proto: 0x920b, nhoff: 34	/* junk */

[   16.898342] Tx: proto: 0x86dd, nhoff: 26	/* ETH_P_IPV6 */
[   16.902705] tag_ops->flow_dissect()
[   16.906227] Tx: proto: 0x920b, nhoff: 34	/* junk */

After:

Rx path (RPS):

[   16.520993] Rx: proto: 0x00f8, nhoff: 0	/* ETH_P_XDSA */
[   16.525260] tag_ops->flow_dissect()
[   16.528808] Rx: proto: 0x0800, nhoff: 8	/* ETH_P_IP */

[   15.484807] Rx: proto: 0x00f8, nhoff: 0	/* ETH_P_XDSA */
[   15.490417] tag_ops->flow_dissect()
[   15.495223] Rx: proto: 0x0806, nhoff: 8	/* ETH_P_ARP */

[   17.134621] Rx: proto: 0x00f8, nhoff: 0	/* ETH_P_XDSA */
[   17.138895] tag_ops->flow_dissect()
[   17.142388] Rx: proto: 0x8100, nhoff: 8	/* ETH_P_8021Q */

Tx path (UP system):

[   15.499558] Tx: proto: 0x0800, nhoff: 26	/* ETH_P_IP */

[   20.664689] Tx: proto: 0x0806, nhoff: 26	/* ETH_P_ARP */

[   18.565782] Tx: proto: 0x86dd, nhoff: 26	/* ETH_P_IPV6 */

In order to fix that we can add the check 'proto == htons(ETH_P_XDSA)'
to prevent code from calling tag_ops->flow_dissect() on Tx.
I also decided to initialize 'offset' variable so tagger callbacks can
now safely leave it untouched without provoking a chaos.

Fixes: 43e66528

 ("net-next: dsa: fix flow dissection")
Signed-off-by: Alexander Lobakin <alobakin@dlink.ru>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>