Re-fix a bug that was fixed long ago but change clobbered by recent merge-o-rama.

Summary:
Re-commit a fix for a bug. This fix was committed a long time ago, but it was
clobbered amidst the recent merge-o-rama. The bug causes a crash (assertion
failure) in the (fallow_cleaning && LRU_reclaim) case. The original fix was
this :

     commit 60a143e4686f5a176e40dee178ed4cc1ceb7c2ba
     Author: Mohan Srinivasan <mohan@fb.com>
     Date:   Mon Jul 9 11:36:54 2012 -0700

     Fix for a crash for the (fallow_cleaning && LRU_reclaim) case.

    Summary: Fix for a copy-paste bug, which can cause crashes when fallow cleaning is enabled in
    conjunction with LRU reclamation. Found by Vyacheslav Maslow and Wang Ning. This bug was also
    fixed a long time ago (but not committed) by Kumar Sundararajan (facebook).

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