block: pass 'op' to blk_queue_enter()

We need to check if the request to be allocated is PREEMPT_ONLY,
and have to pass REQ_PREEEMPT flag to blk_queue_eneter(), so pass
'op' to blk_queue_enter() directly.

Change-Id: I53bafb80d59917f65b5855571489638d9fe507c3
Cc: Bart Van Assche <Bart.VanAssche@wdc.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
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>