Check that the "Can only be attached as root qdisc" error message from
taprio is effective by attempting to attach it to a class of another
taprio qdisc. That operation should fail.

In the bug that was squashed by change "net/sched: taprio: try again to
report q->qdiscs[] to qdisc_leaf()", grafting a child taprio to a root
software taprio would be misinterpreted as a change() to the root
taprio. Catch this by looking at whether the base-time of the root
taprio has changed to follow the base-time of the child taprio,
something which should have absolutely never happened assuming correct
semantics.

Vinicius points out that looking at "base_time" in the tc qdisc show
output is unreliable because user space is in a race with the kernel
applying the setting. So we create a helper bash script which waits
while there is any pending schedule.

Link: https://lore.kernel.org/netdev/87il9w0xx7.fsf@intel.com/Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>
Link: https://lore.kernel.org/r/20230807193324.4128292-11-vladimir.oltean@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>