mount: report . and .. from windows directories (#10556)

* mount: report . and .. from windows directories

WinFsp strips the dot entries for the root itself and expects every other
directory to report them, the way a real NTFS enumeration does: its
dirctl test asserts a subdirectory's first two entries are "." and ".."
and that a hundred files enumerate as 102 entries. Dropping them
unconditionally is what fails querydir_test.

The Go test that guarded the old behaviour went with it: os.File.Readdir
filters dot entries itself, so it could never have observed either way.

* mount: give the windows dot entries their directory type

The readdir fills an attribute block only for real children, so "." and
".." arrived with a zeroed one and were reported with mode 0. Windows
refuses to enumerate a directory whose first entry is not marked as a
directory, which is the assertion querydir_test fails on with
STATUS_OBJECT_NAME_NOT_FOUND.

They now carry the type the readdir already knew. The explorer walk also
names any unexpected entry rather than only counting, so a dot entry
leaking through reads differently from a missing file.

.github/workflows/mount-windows.yml

@@ -165,8 +165,13 @@ jobs:
         Write-Host "::group::explorer-style walk"
         New-Item -ItemType Directory -Force -Path S:\walk | Out-Null
         1..200 | ForEach-Object { Set-Content -Path "S:\walk\f$_.txt" -Value "line $_" }
-        $count = (Get-ChildItem S:\walk | Measure-Object).Count
-        if ($count -ne 200) { throw "listed $count files, expected 200" }
+        $names = @(Get-ChildItem S:\walk | ForEach-Object { $_.Name })
+        if ($names.Count -ne 200) {
+          # Name the strays: a dot entry surfacing here is a different problem
+          # from a missing or duplicated file.
+          $unexpected = $names | Where-Object { $_ -notmatch '^f\d+\.txt$' }
+          throw "listed $($names.Count) entries, expected 200; unexpected: $($unexpected -join ', ')"
+        }
         $body = Get-Content S:\walk\f42.txt
         if ($body -ne 'line 42') { throw "unexpected content: $body" }
         Copy-Item S:\walk\f42.txt S:\walk\copy.txt