dm rq: take request_queue lock while clearing QUEUE_FLAG_STOPPED

Every call of queue_flag_clear_unlocked() after block device
initialization has finished is wrong if blk_cleanup_queue() can be
called concurrently.  Convert queue_flag_clear_unlocked() into
queue_flag_clear() and protect it by the block layer queue lock.

Also, factor out dm_mq_start_queue().

Reported-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org