block: Avoid sleeping function called from invalid context bug

This was caught during QA test:

 BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:942
 in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 243401, name: sed
 INFO: lockdep is turned off.
 Preemption disabled at:
 [<ffffffff89b26268>] blk_cgroup_bio_start+0x28/0xd0

 CPU: 2 PID: 243401 Comm: sed Kdump: loaded Not tainted 4.18.0-353.rt7.138.el8.x86_64+debug #1
 Hardware name: HP ProLiant DL380 Gen9, BIOS P89 05/06/2015
 Call Trace:
  dump_stack+0x5c/0x80
  ___might_sleep.cold.89+0xf5/0x109
  rt_spin_lock+0x3e/0xd0
  ? __blk_add_trace+0x428/0x4b0
  __blk_add_trace+0x428/0x4b0
  blk_add_trace_bio+0x16e/0x1c0
  generic_make_request_checks+0x7e8/0x8c0
  generic_make_request+0x3c/0x420
  ? membarrier_private_expedited+0xd0/0x2b0
  ? lock_release+0x1ca/0x450
  ? submit_bio+0x3c/0x160
  ? _raw_spin_unlock_irqrestore+0x3c/0x80
  submit_bio+0x3c/0x160
  ? rt_mutex_futex_unlock+0x66/0xa0
  iomap_submit_ioend.isra.36+0x4a/0x70
  xfs_vm_writepages+0x65/0x90 [xfs]
  do_writepages+0x41/0xe0
  ? rt_mutex_futex_unlock+0x66/0xa0
  __filemap_fdatawrite_range+0xce/0x110
  xfs_release+0x11c/0x160 [xfs]
  __fput+0xd5/0x270
  task_work_run+0xa1/0xd0
  exit_to_usermode_loop+0x14d/0x160
  do_syscall_64+0x23b/0x240
  entry_SYSCALL_64_after_hwframe+0x6a/0xdf

We replace the get/put_cpu() call by get/put_cpu_light to avoid this bug.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>

block/blk-cgroup.c

@@ -1912,7 +1912,7 @@ void blk_cgroup_bio_start(struct bio *bio)
 	struct blkg_iostat_set *bis;
 	unsigned long flags;
 
-	cpu = get_cpu();
+	cpu = get_cpu_light();
 	bis = per_cpu_ptr(bio->bi_blkg->iostat_cpu, cpu);
 	flags = u64_stats_update_begin_irqsave(&bis->sync);
 
@@ -1929,7 +1929,7 @@ void blk_cgroup_bio_start(struct bio *bio)
 	u64_stats_update_end_irqrestore(&bis->sync, flags);
 	if (cgroup_subsys_on_dfl(io_cgrp_subsys))
 		cgroup_rstat_updated(bio->bi_blkg->blkcg->css.cgroup, cpu);
-	put_cpu();
+	put_cpu_light();
 }
 
 static int __init blkcg_init(void)