blk-mq: return if queue is frozen via current blk_freeze_queue_start

We need this return value in the following patch to decide
if a explicit synchronize_rcu() is needed.

Change-Id: I75559b7ec1dee4df607fd3e67abc6954b656edf4
Cc: Bart Van Assche <Bart.VanAssche@wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Patch-mainline: linux-block@vger.kernel.org @ 03/10/2017, 22:04
Signed-off-by: Pradeep P V K <ppvk@codeaurora.org>