block: Add the QUEUE_FLAG_PREEMPT_ONLY request queue flag

block: Add the QUEUE_FLAG_PREEMPT_ONLY request queue flag

This flag will be used in the next patch to let the block layer
core know whether or not a SCSI request queue has been quiesced.
A quiesced SCSI queue namely only processes RQF_PREEMPT requests.

Signed-off-by: Bart Van Assche <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Tested-by: Martin Steigerwald <[email protected]>
Tested-by: Oleksandr Natalenko <[email protected]>
Cc: Ming Lei <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>