net: dsa: Make deferred_xmit private to sja1105

There are 3 things that are wrong with the DSA deferred xmit mechanism:

1. Its introduction has made the DSA hotpath ever so slightly more
   inefficient for everybody, since DSA_SKB_CB(skb)->deferred_xmit needs
   to be initialized to false for every transmitted frame, in order to
   figure out whether the driver requested deferral or not (a very rare
   occasion, rare even for the only driver that does use this mechanism:
   sja1105). That was necessary to avoid kfree_skb from freeing the skb.

2. Because L2 PTP is a link-local protocol like STP, it requires
   management routes and deferred xmit with this switch. But as opposed
   to STP, the deferred work mechanism needs to schedule the packet
   rather quickly for the TX timstamp to be collected in time and sent
   to user space. But there is no provision for controlling the
   scheduling priority of this deferred xmit workqueue. Too bad this is
   a rather specific requirement for a feature that nobody else uses
   (more below).

3. Perhaps most importantly, it makes the DSA core adhere a bit too
   much to the NXP company-wide policy "Innovate Where It Doesn't
   Matter". The sja1105 is probably the only DSA switch that requires
   some frames sent from the CPU to be routed to the slave port via an
   out-of-band configuration (register write) rather than in-band (DSA
   tag). And there are indeed very good reasons to not want to do that:
   if that out-of-band register is at the other end of a slow bus such
   as SPI, then you limit that Ethernet flow's throughput to effectively
   the throughput of the SPI bus. So hardware vendors should definitely
   not be encouraged to design this way. We do _not_ want more
   widespread use of this mechanism.

Luckily we have a solution for each of the 3 issues:

For 1, we can just remove that variable in the skb->cb and counteract
the effect of kfree_skb with skb_get, much to the same effect. The
advantage, of course, being that anybody who doesn't use deferred xmit
doesn't need to do any extra operation in the hotpath.

For 2, we can create a kernel thread for each port's deferred xmit work.
If the user switch ports are named swp0, swp1, swp2, the kernel threads
will be named swp0_xmit, swp1_xmit, swp2_xmit (there appears to be a 15
character length limit on kernel thread names). With this, the user can
change the scheduling priority with chrt $(pidof swp2_xmit).

For 3, we can actually move the entire implementation to the sja1105
driver.

So this patch deletes the generic implementation from the DSA core and
adds a new one, more adequate to the requirements of PTP TX
timestamping, in sja1105_main.c.

Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>