blk_queue_enter() accepts BLK_MQ_REQ_PM requests independent of the runtime
power management state. Now that SCSI domain validation no longer depends
on this behavior, modify the behavior of blk_queue_enter() as follows:

   - Do not accept any requests while suspended.

   - Only process power management requests while suspending or resuming.

Submitting BLK_MQ_REQ_PM requests to a device that is runtime suspended
causes runtime-suspended devices not to resume as they should. The request
which should cause a runtime resume instead gets issued directly, without
resuming the device first. Of course the device can't handle it properly,
the I/O fails, and the device remains suspended.

The problem is fixed by checking that the queue's runtime-PM status isn't
RPM_SUSPENDED before allowing a request to be issued, and queuing a
runtime-resume request if it is.  In particular, the inline
blk_pm_request_resume() routine is renamed blk_pm_resume_queue() and the
code is unified by merging the surrounding checks into the routine.  If the
queue isn't set up for runtime PM, or there currently is no restriction on
allowed requests, the request is allowed.  Likewise if the BLK_MQ_REQ_PM
flag is set and the status isn't RPM_SUSPENDED.  Otherwise a runtime resume
is queued and the request is blocked until conditions are more suitable.

[ bvanassche: modified commit message and removed Cc: stable because
  without the previous patches from this series this patch would break
  parallel SCSI domain validation + introduced queue_rpm_status() ]

Link: https://lore.kernel.org/r/20201209052951.16136-9-bvanassche@acm.org
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Can Guo <cang@codeaurora.org>
Cc: Stanley Chu <stanley.chu@mediatek.com>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reported-and-tested-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>