iolatency_clear_scaling() updates child_lat.lock with hardirqs enabled.
The bio completion path can take the same lock from hardirq context.

This triggers lockdep after io.latency is configured and I/O completes.
Full lockdep report:

  WARNING: inconsistent lock state
  7.1.0-rc2-g6a04b2279273 #1 Not tainted
  --------------------------------
  inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.
  swapper/0/0 [HC1[1]:SC0[0]:HE0:SE1] takes:
  ffff88810c682d10 (&iolat->child_lat.lock){?.+.}-{3:3}, at: blkcg_iolatency_done_bio+0x6e7/0xb90
  {HARDIRQ-ON-W} state was registered at:
    lock_acquire+0xd4/0x290
    _raw_spin_lock+0x3a/0x70
    iolatency_set_limit+0x49b/0x590
    cgroup_file_write+0x1c5/0x4b0
    kernfs_fop_write_iter+0x1d7/0x280
    vfs_write+0x580/0x630
    ksys_write+0xec/0x190
    do_syscall_64+0x156/0x490
    entry_SYSCALL_64_after_hwframe+0x77/0x7f
  irq event stamp: 328476
  hardirqs last  enabled at (328475): [<ffffffffa4dd93b1>] do_idle+0x261/0x400
  hardirqs last disabled at (328476): [<ffffffffa68347f3>] common_interrupt+0x13/0x90
  softirqs last  enabled at (328398): [<ffffffffa4d508ac>] __irq_exit_rcu+0x8c/0x150
  softirqs last disabled at (328387): [<ffffffffa4d508ac>] __irq_exit_rcu+0x8c/0x150

                            other info that might help us debug this:
   Possible unsafe locking scenario:

         CPU0
         ----
    lock(&iolat->child_lat.lock);
    <Interrupt>
      lock(&iolat->child_lat.lock);

                             *** DEADLOCK ***

  1 lock held by swapper/0/0:
   #0: ffff888103365450 (&virtscsi_vq->vq_lock){-.-.}-{3:3}, at: virtscsi_vq_done+0x9f/0x130

                            stack backtrace:
  CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 7.1.0-rc2-g6a04b2279273 #1 PREEMPT  1c49bdb9e32f352d2b66a5ca23d36d656c610458
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-5.fc42 04/01/2014
  Call Trace:
   <IRQ>
   dump_stack_lvl+0x54/0x70
   print_usage_bug+0x26d/0x280
   mark_lock_irq+0x3ef/0x400
   ? save_trace+0x3d/0x2f0
   ? __pfx_stack_trace_consume_entry+0x10/0x10
   mark_lock+0x117/0x190
   __lock_acquire+0x570/0x2850
   ? stack_trace_save+0xa1/0xe0
   ? __pfx_stack_trace_save+0x10/0x10
   ? filter_irq_stacks+0x27/0x80
   ? stack_depot_save_flags+0x32/0x7f0
   lock_acquire+0xd4/0x290
   ? blkcg_iolatency_done_bio+0x6e7/0xb90
   ? kvm_sched_clock_read+0x11/0x20
   ? local_clock_noinstr+0xc/0xc0
   ? local_clock+0x15/0x30
   ? lock_release+0x111/0x470
   ? blkcg_iolatency_done_bio+0x6e7/0xb90
   _raw_spin_lock_irqsave+0x4c/0x90
   ? blkcg_iolatency_done_bio+0x6e7/0xb90
   blkcg_iolatency_done_bio+0x6e7/0xb90
   ? __pfx_blkcg_iolatency_done_bio+0x10/0x10
   __rq_qos_done_bio+0x51/0x60
   bio_endio+0x135/0x320
   blk_update_request+0x1e6/0x570
   scsi_end_request+0x4b/0x410
   scsi_io_completion+0x83/0x170
   ? __pfx_virtscsi_complete_cmd+0x10/0x10
   virtscsi_vq_done+0xd7/0x130
   ? lock_acquire+0xd4/0x290
   ? __pfx_virtscsi_vq_done+0x10/0x10
   ? local_clock_noinstr+0xc/0xc0
   ? local_clock+0x15/0x30
   vring_interrupt+0x13b/0x150
   ? __pfx_vring_interrupt+0x10/0x10
   __handle_irq_event_percpu+0x145/0x4b0
   handle_irq_event+0x54/0xb0
   handle_edge_irq+0x111/0x320
   __common_interrupt+0x97/0xf0
   common_interrupt+0x7e/0x90
   </IRQ>
   <TASK>
   asm_common_interrupt+0x26/0x40
  RIP: 0010:pv_native_safe_halt+0x13/0x20
  Code: d3 a5 01 00 cc 66 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 66 90 0f 00 2d 2f 39 21 00 f3 0f 1e fa fb f4 <c3> cc cc cc cc cc 0f 1f 80 00 00 00 00 90 90 90 90 90 90 90 90 90
  RSP: 0018:ffffffffa7607e00 EFLAGS: 00000246
  RAX: 000000000005031b RBX: ffffffffa4dd93b1 RCX: ffffffffa683884b
  RDX: 0000000000000001 RSI: 0000000000000004 RDI: ffffffffa4dd93b1
  RBP: ffffffffa7607ed0 R08: ffff888117bf408b R09: 1ffff11022f7e811
  R10: dffffc0000000000 R11: ffffed1022f7e812 R12: 0000000000000000
  R13: 0000000000000000 R14: 0000000000000000 R15: ffffffffa7f6cff0
   ? do_idle+0x261/0x400
   ? ct_kernel_exit+0xcb/0x110
   ? do_idle+0x261/0x400
   default_idle+0x9/0x20
   default_idle_call+0x73/0xb0
   do_idle+0x261/0x400
   ? __pfx_do_idle+0x10/0x10
   ? local_clock_noinstr+0x30/0xc0
   ? local_clock+0x15/0x30
   cpu_startup_entry+0x36/0x40
   rest_init+0x207/0x210
   start_kernel+0x321/0x370
   x86_64_start_reservations+0x24/0x30
   x86_64_start_kernel+0x13a/0x140
   common_startup_64+0x13e/0x147
   </TASK>

Fix it by using spin_lock_irqsave() in iolatency_clear_scaling().
Use irqsave rather than spin_lock_irq() because the same helper is also
called from pd_offline_fn paths where hardirqs can already be disabled
by blkcg teardown/deactivation locks. spin_unlock_irq() would wrongly
enable hardirqs in those paths.

Fixes: d706751 ("block: introduce blk-iolatency io controller")
Signed-off-by: Yu Kuai <yukuai@fygo.io>