virtio_pmem: add the missing REQ_OP_WRITE for flush bio

When doing mkfs.xfs on a pmem device, the following warning was
reported:

 ------------[ cut here ]------------
 WARNING: CPU: 2 PID: 384 at block/blk-core.c:751 submit_bio_noacct
 Modules linked in:
 CPU: 2 PID: 384 Comm: mkfs.xfs Not tainted 6.4.0-rc7+ #154
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
 RIP: 0010:submit_bio_noacct+0x340/0x520
 ......
 Call Trace:
  <TASK>
  ? submit_bio_noacct+0xd5/0x520
  submit_bio+0x37/0x60
  async_pmem_flush+0x79/0xa0
  nvdimm_flush+0x17/0x40
  pmem_submit_bio+0x370/0x390
  __submit_bio+0xbc/0x190
  submit_bio_noacct_nocheck+0x14d/0x370
  submit_bio_noacct+0x1ef/0x520
  submit_bio+0x55/0x60
  submit_bio_wait+0x5a/0xc0
  blkdev_issue_flush+0x44/0x60

The root cause is that submit_bio_noacct() needs bio_op() is either
WRITE or ZONE_APPEND for flush bio and async_pmem_flush() doesn't assign
REQ_OP_WRITE when allocating flush bio, so submit_bio_noacct just fail
the flush bio.

Simply fix it by adding the missing REQ_OP_WRITE for flush bio. And we
could fix the flush order issue and do flush optimization later.

Cc: stable@vger.kernel.org # 6.3+
Fixes: b4a6bb3 ("block: add a sanity check for non-write flush/fua bios")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Tested-by: Pankaj Gupta <pankaj.gupta@amd.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

drivers/nvdimm/nd_virtio.c

@@ -105,7 +105,8 @@ int async_pmem_flush(struct nd_region *nd_region, struct bio *bio)
 	 * parent bio. Otherwise directly call nd_region flush.
 	 */
 	if (bio && bio->bi_iter.bi_sector != -1) {
-		struct bio *child = bio_alloc(bio->bi_bdev, 0, REQ_PREFLUSH,
+		struct bio *child = bio_alloc(bio->bi_bdev, 0,
+					      REQ_OP_WRITE | REQ_PREFLUSH,
 					      GFP_ATOMIC);
 
 		if (!child)