virtio_pmem: add the missing REQ_OP_WRITE for flush bio

In the Linux kernel, the following vulnerability has been resolved:

virtio_pmem: add the missing REQ_OP_WRITE for flush bio

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

------------[ 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.