virtio-blk: support completion batching for the IRQ path

This patch adds completion batching to the IRQ path. It reuses batch
completion code of virtblk_poll(). It collects requests to io_comp_batch
and processes them all at once. It can boost up the performance by 2%.

To validate the performance improvement and stabilty, I did fio test with
4 vCPU VM and 12 vCPU VM respectively. Both VMs have 8GB ram and the same
number of HW queues as vCPU.
The fio cammad is as follows and I ran the fio 5 times and got IOPS average.
(io_uring, randread, direct=1, bs=512, iodepth=64 numjobs=2,4)

Test result shows about 2% improvement.

           4 vcpu VM       |   numjobs=2   |   numjobs=4
      -----------------------------------------------------------
        fio without patch  |  367.2K IOPS  |   397.6K IOPS
      -----------------------------------------------------------
        fio with patch     |  372.8K IOPS  |   407.7K IOPS

           12 vcpu VM      |   numjobs=2   |   numjobs=4
      -----------------------------------------------------------
        fio without patch  |  363.6K IOPS  |   374.8K IOPS
      -----------------------------------------------------------
        fio with patch     |  373.8K IOPS  |   385.3K IOPS

Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Message-Id: <20221221145456.281218-3-suwan.kim027@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>