fix yet another crash with sliver freeing. This time is due to read-ahead thread racing with remove_all.  This is another reason to remove readahead on sliod.  iozone -a and kernel untar work fine.
fix yet another crash with sliver freeing. This time is due to read-ahead thread racing with
remove_all.  This is another reason to remove readahead on sliod.  iozone -a and kernel untar
work fine.

pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable