There are at most bdev->reqs[VIRTIO_BLK_QUEUE_SIZE] outstanding requests
at any time.  We can simply use the head of each request to fetch the
right 'struct blk_dev_req' in bdev->reqs[].

So, we can eliminate the list and lock operations which introduced by
virtio_blk_req_{pop, push}.

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>