blk-mq: add one API for waiting until quiesce is done

blk-mq: add one API for waiting until quiesce is done

Some drivers(NVMe, SCSI) need to call quiesce and unquiesce in pair, but it
is hard to switch to this style, so these drivers need one atomic flag for
helping to balance quiesce and unquiesce.

When quiesce is in-progress, the driver still needs to wait until
the quiesce is done, so add API of blk_mq_wait_quiesce_done() for
these drivers.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20211109071144.181581-2-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>