sched/topology: Add a cpus_read_lock to rebuild_sched_domains()

A read from /proc/sys/kernel/sched_rt_runtime_us leads to backtrace due
to missing cpu_hotplug_lock with CONFIG_CPUSETS=n. The callchain is
sched_rt_handler() -> partition_sched_domains() -> sched_cache_set() ->
static_key_enable_cpuslocked(&sched_cache_present).

sched_cache_set() itself is also invoked from sched_init_domains() which
is early during the boot, holding just the sched_domains_mutex_lock().
Here is no warning because it happens before user space is running (and
hotplug operations are not possible).

There is also sched_cache_active_set() which acquires the hotplug lock
before invoking any of the _cpuslocked() functions.

This is only a problem with CONFIG_CPUSETS=n because in the =y case the
other implementation of rebuild_sched_domains acquires the CPU-hotplug
lock.

Acquire CPU hotplug lock before in rebuild_sched_domains(), before
partition_sched_domains() is invoked for the CONFIG_CPUSETS=n case.

Fixes: a7660ce159 ("sched/cache: Fix has_multi_llcs iff at least one partition has multiple LLCs")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reivewed-by: Ridong Chen <ridong.chen@linux.dev>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: Aaron Tomlin <atomlin@atomlin.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://patch.msgid.link/20260813073855.ji2UrtVh@linutronix.de

include/linux/cpuset.h

@@ -279,6 +279,7 @@ static inline void dl_rebuild_rd_accounting(void)
 
 static inline void rebuild_sched_domains(void)
 {
+	guard(cpus_read_lock)();
 	partition_sched_domains(1, NULL, NULL);
 }