perf: Fix missing RCU reader protection in perf_event_clear_cpumask()

perf: Fix missing RCU reader protection in perf_event_clear_cpumask()

Running rcutorture scenario TREE05, the below warning is triggered.

[   32.604594] WARNING: suspicious RCU usage
[   32.605928] 6.11.0-rc5-00040-g4ba4f1afb6a9 #55238 Not tainted
[   32.607812] -----------------------------
[   32.609140] kernel/events/core.c:13946 RCU-list traversed in non-reader section!!
[   32.611595] other info that might help us debug this:
[   32.614247] rcu_scheduler_active = 2, debug_locks = 1
[   32.616392] 3 locks held by cpuhp/4/35:
[   32.617687]  #0: ffffffffb666a650 (cpu_hotplug_lock){++++}-{0:0}, at: cpuhp_thread_fun+0x4e/0x200
[   32.620563]  #1: ffffffffb666cd20 (cpuhp_state-down){+.+.}-{0:0}, at: cpuhp_thread_fun+0x4e/0x200
[   32.623412]  #2: ffffffffb677c288 (pmus_lock){+.+.}-{3:3}, at: perf_event_exit_cpu_context+0x32/0x2f0

In perf_event_clear_cpumask(), uses list_for_each_entry_rcu() without an
obvious RCU read-side critical section.

Either pmus_srcu or pmus_lock is good enough to protect the pmus list.
In the current context, pmus_lock is already held. The
list_for_each_entry_rcu() is not required.

Fixes: 4ba4f1afb6a9 ("perf: Generic hotplug support for a PMU with a scope")
Closes: https://lore.kernel.org/lkml/2b66dff8-b827-494b-b151-1ad8d56f13e6@paulmck-laptop/
Closes: https://lore.kernel.org/oe-lkp/[email protected]
Reported-by: "Paul E. McKenney" <[email protected]>
Reported-by: kernel test robot <[email protected]>
Suggested-by: Peter Zijlstra <[email protected]>
Signed-off-by: Kan Liang <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Tested-by: "Paul E. McKenney" <[email protected]>
Link: https://lore.kernel.org/r/[email protected]