net/sched: remove qdisc_root_lock() helper

the last caller has been removed with commit 96f5e66 ("mac80211: fix
aggregation for hardware with ampdu queues"), so it's safe to remove this
function.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Link: https://lore.kernel.org/r/703d549e3088367651d92a059743f1be848d74b7.1658133689.git.dcaratti@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/sch_generic.h

@@ -551,25 +551,6 @@ static inline struct Qdisc *qdisc_root_sleeping(const struct Qdisc *qdisc)
 	return qdisc->dev_queue->qdisc_sleeping;
 }
 
-/* The qdisc root lock is a mechanism by which to top level
- * of a qdisc tree can be locked from any qdisc node in the
- * forest.  This allows changing the configuration of some
- * aspect of the qdisc tree while blocking out asynchronous
- * qdisc access in the packet processing paths.
- *
- * It is only legal to do this when the root will not change
- * on us.  Otherwise we'll potentially lock the wrong qdisc
- * root.  This is enforced by holding the RTNL semaphore, which
- * all users of this lock accessor must do.
- */
-static inline spinlock_t *qdisc_root_lock(const struct Qdisc *qdisc)
-{
-	struct Qdisc *root = qdisc_root(qdisc);
-
-	ASSERT_RTNL();
-	return qdisc_lock(root);
-}
-
 static inline spinlock_t *qdisc_root_sleeping_lock(const struct Qdisc *qdisc)
 {
 	struct Qdisc *root = qdisc_root_sleeping(qdisc);