TestUsage broken on ZFS

When I run `TestUsage` in the `fs` package with a temp dir on a ZFS mount, it fails like this:

```
$ TMPDIR=/testpool/tmp go test ./fs -v -run TestUsage
=== RUN TestUsage
=== RUN TestUsage/SingleSmallFile
=== PAUSE TestUsage/SingleSmallFile
=== RUN TestUsage/MultipleSmallFile
=== PAUSE TestUsage/MultipleSmallFile
=== RUN TestUsage/BiggerFiles
=== PAUSE TestUsage/BiggerFiles
=== RUN TestUsage/SparseFiles
=== PAUSE TestUsage/SparseFiles
=== RUN TestUsage/Hardlinks
=== PAUSE TestUsage/Hardlinks
=== RUN TestUsage/HardlinkSparefile
=== PAUSE TestUsage/HardlinkSparefile
=== CONT TestUsage/SingleSmallFile
du_test.go:129: Wrong usage size 1536, expected 132096
=== CONT TestUsage/HardlinkSparefile
=== CONT TestUsage/SparseFiles
du_test.go:129: Wrong usage size 2560, expected 525312
=== CONT TestUsage/MultipleSmallFile
=== CONT TestUsage/Hardlinks
=== CONT TestUsage/BiggerFiles
=== CONT TestUsage/HardlinkSparefile
du_test.go:129: Wrong usage size 1536, expected 263168
=== CONT TestUsage/MultipleSmallFile
du_test.go:129: Wrong usage size 2048, expected 263168
=== CONT TestUsage/Hardlinks
du_test.go:129: Wrong usage size 1536, expected 132096
=== CONT TestUsage/BiggerFiles
du_test.go:129: Wrong usage size 2560, expected 394240
--- FAIL: TestUsage (0.00s)
--- FAIL: TestUsage/SingleSmallFile (0.00s)
--- FAIL: TestUsage/SparseFiles (0.00s)
--- FAIL: TestUsage/Hardlinks (0.00s)
--- FAIL: TestUsage/BiggerFiles (0.00s)
--- FAIL: TestUsage/HardlinkSparefile (0.00s)
--- FAIL: TestUsage/MultipleSmallFile (0.00s)
FAIL
FAIL github.com/containerd/continuity/fs 0.006s
FAIL
```

If I add a sleep before `DiskUsage` is called, I get slightly different but still-failing results:

```
$ TMPDIR=/testpool/tmp go test ./fs -v -run TestUsage
=== RUN TestUsage
=== RUN TestUsage/SingleSmallFile
=== PAUSE TestUsage/SingleSmallFile
=== RUN TestUsage/MultipleSmallFile
=== PAUSE TestUsage/MultipleSmallFile
=== RUN TestUsage/BiggerFiles
=== PAUSE TestUsage/BiggerFiles
=== RUN TestUsage/SparseFiles
=== PAUSE TestUsage/SparseFiles
=== RUN TestUsage/Hardlinks
=== PAUSE TestUsage/Hardlinks
=== RUN TestUsage/HardlinkSparefile
=== PAUSE TestUsage/HardlinkSparefile
=== CONT TestUsage/SingleSmallFile
=== CONT TestUsage/HardlinkSparefile
=== CONT TestUsage/Hardlinks
=== CONT TestUsage/SparseFiles
=== CONT TestUsage/SingleSmallFile
du_test.go:132: Wrong usage size 2048, expected 132096
=== CONT TestUsage/HardlinkSparefile
du_test.go:132: Wrong usage size 266752, expected 263168
=== CONT TestUsage/BiggerFiles
=== CONT TestUsage/MultipleSmallFile
=== CONT TestUsage/Hardlinks
du_test.go:132: Wrong usage size 62976, expected 132096
=== CONT TestUsage/SparseFiles
du_test.go:132: Wrong usage size 399360, expected 525312
=== CONT TestUsage/BiggerFiles
du_test.go:132: Wrong usage size 55296, expected 394240
=== CONT TestUsage/MultipleSmallFile
du_test.go:132: Wrong usage size 3072, expected 263168
--- FAIL: TestUsage (0.00s)
--- FAIL: TestUsage/SingleSmallFile (5.00s)
--- FAIL: TestUsage/HardlinkSparefile (5.00s)
--- FAIL: TestUsage/Hardlinks (5.00s)
--- FAIL: TestUsage/SparseFiles (5.00s)
--- FAIL: TestUsage/BiggerFiles (5.00s)
--- FAIL: TestUsage/MultipleSmallFile (5.00s)
FAIL
FAIL github.com/containerd/continuity/fs 10.005s
FAIL
```

It looks like the `DiskUsage` function is working correctly, but the expected size calculation doesn't hold for how ZFS manages its blocks. This occurs both with OpenZFS on Ubuntu 16.04 and on FreeBSD 12.