net: dsa: tag_ocelot: break circular dependency with ocelot switch lib driver

net: dsa: tag_ocelot: break circular dependency with ocelot switch lib driver

As explained here:
https://lore.kernel.org/netdev/20210908220834.d7gmtnwrorhharna@skbuf/
DSA tagging protocol drivers cannot depend on symbols exported by switch
drivers, because this creates a circular dependency that breaks module
autoloading.

The tag_ocelot.c file depends on the ocelot_ptp_rew_op() function
exported by the common ocelot switch lib. This function looks at
OCELOT_SKB_CB(skb) and computes how to populate the REW_OP field of the
DSA tag, for PTP timestamping (the command: one-step/two-step, and the
TX timestamp identifier).

None of that requires deep insight into the driver, it is quite
stateless, as it only depends upon the skb->cb. So let's make it a
static inline function and put it in include/linux/dsa/ocelot.h, a
file that despite its name is used by the ocelot switch driver for
populating the injection header too - since commit 40d3f295b5fe ("net:
mscc: ocelot: use common tag parsing code with DSA").

With that function declared as static inline, its body is expanded
inside each call site, so the dependency is broken and the DSA tagger
can be built without the switch library, upon which the felix driver
depends.

Fixes: 39e5308b3250 ("net: mscc: ocelot: support PTP Sync one-step timestamping")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>