block: Change a rcu_read_{lock,unlock}_sched() pair into rcu_read_{lock,unlock}()

scsi_device_quiesce() uses synchronize_rcu() to guarantee that the
effect of blk_set_preempt_only() will be visible for percpu_ref_tryget()
calls that occur after the queue unfreeze by using the approach
explained in https://lwn.net/Articles/573497/. The rcu read lock and
unlock calls in blk_queue_enter() form a pair with the synchronize_rcu()
call in scsi_device_quiesce(). Both scsi_device_quiesce() and
blk_queue_enter() must either use regular RCU or RCU-sched.
Since neither the RCU-protected code in blk_queue_enter() nor
blk_queue_usage_counter_release() sleeps, regular RCU protection
is sufficient. Note: scsi_device_quiesce() does not have to be
modified since it already uses synchronize_rcu().

Reported-by: Tejun Heo <[email protected]>
Fixes: 3a0a529971ec ("block, scsi: Make SCSI quiesce and resume work reliably")
Signed-off-by: Bart Van Assche <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: Ming Lei <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Cc: Oleksandr Natalenko <[email protected]>
Cc: Martin Steigerwald <[email protected]>
Cc: [email protected] # v4.15
Signed-off-by: Jens Axboe <[email protected]>

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