Bug 572018 - Upgrading from 1.2.5 to 1.2.6.a2 deletes userRoot
    
    https://bugzilla.redhat.com/show_bug.cgi?id=572018
    
    Description: The upgrade tool dn2rdn shares the reindex code, in
    which a cursor is set to each entry in id2entry.db# converting
    dn to rdn and put it into the btree with db->put one by one.
    It turned out the strategy does not work for the case the btree
    structure is modified.  In that case, the btree could get corrupted.
    This patch creates a new id2entry_tmp.db# from the scratch.  Once
    all the entries are successfully converted, the old db is removed
    and id2entry_tmp.db# is renamed to id2entry.db#.