net/sched: taprio: centralize mqprio qopt validation

There is a lot of code in taprio which is "borrowed" from mqprio.
It makes sense to put a stop to the "borrowing" and start actually
reusing code.

Because taprio and mqprio are built as part of different kernel modules,
code reuse can only take place either by writing it as static inline
(limiting), putting it in sch_generic.o (not generic enough), or
creating a third auto-selectable kernel module which only holds library
code. I opted for the third variant.

In a previous change, mqprio gained support for reverse TC:TXQ mappings,
something which taprio still denies. Make taprio use the same validation
logic so that it supports this configuration as well.

The taprio code didn't enforce TXQ overlaps in txtime-assist mode and
that looks intentional, even if I've no idea why that might be. Preserve
that, but add a comment.

There isn't any dedicated MAINTAINERS entry for mqprio, so nothing to
update there.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/Kconfig

@@ -195,8 +195,14 @@ config NET_SCH_ETF
 	  To compile this code as a module, choose M here: the
 	  module will be called sch_etf.
 
+config NET_SCH_MQPRIO_LIB
+	tristate
+	help
+	  Common library for manipulating mqprio queue configurations.
+
 config NET_SCH_TAPRIO
 	tristate "Time Aware Priority (taprio) Scheduler"
+	select NET_SCH_MQPRIO_LIB
 	help
 	  Say Y here if you want to use the Time Aware Priority (taprio) packet
 	  scheduling algorithm.
@@ -253,6 +259,7 @@ config NET_SCH_DRR
 
 config NET_SCH_MQPRIO
 	tristate "Multi-queue priority scheduler (MQPRIO)"
+	select NET_SCH_MQPRIO_LIB
 	help
 	  Say Y here if you want to use the Multi-queue Priority scheduler.
 	  This scheduler allows QOS to be offloaded on NICs that have support