runtime: correct counters in sweep

runtime: correct counters in sweep

    In the sweep code we can sometimes see incorrect counts when
    conservative stack scanning causes us to grey an object that we
    earlier decided could be freed.  We already ignored this check, but
    adjust this case to maintain correct span counts when it happens.
    This gives us slightly more correct numbers in MemStats, and helps
    avoid a rare failure in TestReadMemStats.

    Also fix the free index, and cope with finding a full span when
    allocating a new one.

    Reviewed-on: https://go-review.googlesource.com/134216

From-SVN: r264294