virtio-blk: Fix double completion for werror=stop

When a request R is absorbed by request M, it is appended to the
"mr_next" queue led by M, and is completed together with the completion
of M, in virtio_blk_rw_complete.

During DMA restart in virtio_blk_dma_restart_bh, requests in s->rq are
parsed and submitted again, possibly with a stale req->mr_next. It could
be a problem if the request merging in virtio_blk_handle_request hasn't
refreshed every mr_next pointer, in which case, virtio_blk_rw_complete
could walk through unexpected requests following the stale pointers.

Fix this by unsetting the pointer in virtio_blk_rw_complete. It is safe
because this req is either completed and freed right away, or it will be
restarted and parsed from scratch out of the vq later.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
