LU-1867 tests: replay-single test_89: @@ FAIL: 4 blocks leaked

LU-1867 tests: replay-single test_89: @@ FAIL: 4 blocks leaked

The config llog data will changed while OST remount. It may
impact 'df' output. So we should allow extra one block size
different(4k).

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I663685d55a5a31af48d541e08e9e97fed2b8e25c
Reviewed-on: http://review.whamcloud.com/4130
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>