kmemleak: avoid RCU stalls when freeing metadata for per-CPU pointers

On systems with large number of CPUs, the following soft lockup splat
might sometimes happen:

[ 2656.001617] watchdog: BUG: soft lockup - CPU#364 stuck for 21s! [ksoftirqd/364:2206]
  :
[ 2656.141194] RIP: 0010:_raw_spin_unlock_irqrestore+0x3d/0x70
  :
 2656.241214] Call Trace:
[ 2656.243971]  <IRQ>
[ 2656.246237]  ? show_trace_log_lvl+0x1c4/0x2df
[ 2656.251152]  ? show_trace_log_lvl+0x1c4/0x2df
[ 2656.256066]  ? kmemleak_free_percpu+0x11f/0x1f0
[ 2656.261173]  ? watchdog_timer_fn+0x379/0x470
[ 2656.265984]  ? __pfx_watchdog_timer_fn+0x10/0x10
[ 2656.271179]  ? __hrtimer_run_queues+0x5f3/0xd00
[ 2656.276283]  ? __pfx___hrtimer_run_queues+0x10/0x10
[ 2656.281783]  ? ktime_get_update_offsets_now+0x95/0x2c0
[ 2656.287573]  ? ktime_get_update_offsets_now+0xdd/0x2c0
[ 2656.293380]  ? hrtimer_interrupt+0x2e9/0x780
[ 2656.298221]  ? __sysvec_apic_timer_interrupt+0x184/0x640
[ 2656.304211]  ? sysvec_apic_timer_interrupt+0x8e/0xc0
[ 2656.309807]  </IRQ>
[ 2656.312169]  <TASK>
[ 2656.326110]  kmemleak_free_percpu+0x11f/0x1f0
[ 2656.331015]  free_percpu.part.0+0x1b/0xe70
[ 2656.335635]  free_vfsmnt+0xb9/0x100
[ 2656.339567]  rcu_do_batch+0x3c8/0xe30
[ 2656.363693]  rcu_core+0x3de/0x5a0
[ 2656.367433]  __do_softirq+0x2d0/0x9a8
[ 2656.381119]  run_ksoftirqd+0x36/0x60
[ 2656.385145]  smpboot_thread_fn+0x556/0x910
[ 2656.394971]  kthread+0x2a4/0x350
[ 2656.402826]  ret_from_fork+0x29/0x50
[ 2656.406861]  </TASK>

The issue is caused by kmemleak registering each per_cpu_ptr()
corresponding to the __percpu pointer.  This is unnecessary since such
individual per-CPU pointers are not tracked anyway.  Create a new
object_percpu_tree_root rbtree that stores a single __percpu pointer
together with an OBJECT_PERCPU flag for the kmemleak metadata.  Scanning
needs to be done for all per_cpu_ptr() pointers with a cond_resched()
between each CPU iteration to avoid RCU stalls.

[catalin.marinas@arm.com: update comment]
  Link: https://lkml.kernel.org/r/20231206114414.2085824-1-catalin.marinas@arm.com
  Link: https://lore.kernel.org/r/20231127194153.289626-1-longman@redhat.comLink: https://lkml.kernel.org/r/20231201190829.825856-1-catalin.marinas@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Waiman Long <longman@redhat.com>
Closes: https://lore.kernel.org/r/20231127194153.289626-1-longman@redhat.comReviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>