virtio_pmem: add the missing REQ_OP_WRITE for flush bio

Summary

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.