nvdimm: virtio_pmem: serialize flush requests

nvdimm: virtio_pmem: serialize flush requests

Under heavy concurrent flush traffic, virtio-pmem can overflow its request
virtqueue (req_vq): virtqueue_add_sgs() starts returning -ENOSPC and the
driver logs "no free slots in the virtqueue". Shortly after that the
device enters VIRTIO_CONFIG_S_NEEDS_RESET and flush requests fail with
"virtio pmem device needs a reset".

Serialize virtio_pmem_flush() with a per-device mutex so only one flush
request is in-flight at a time. This prevents req_vq descriptor overflow
under high concurrency.

Reproducer (guest with virtio-pmem):
  - mkfs.ext4 -F /dev/pmem0
  - mount -t ext4 -o dax,noatime /dev/pmem0 /mnt/bench
  - fio: ioengine=io_uring rw=randwrite bs=4k iodepth=64 numjobs=64
        direct=1 fsync=1 runtime=30s time_based=1
  - dmesg: "no free slots in the virtqueue"
           "virtio pmem device needs a reset"

Fixes: 6e84200c0a29 ("virtio-pmem: Add virtio pmem driver")
Signed-off-by: Li Chen <me@linux.beauty>
Acked-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20260203021353.121091-1-me@linux.beauty
Signed-off-by: Ira Weiny <ira.weiny@intel.com>