azurefiles: fix incorrect modtime after uploading a file or setting its modtime

Update and SetModTime kept the caller's full nanosecond precision
modtime in memory while the server stores SMB LastWriteTime with
100 ns (FILETIME) precision, so until the object was re-read the
modtime did not match what a fresh listing would report. This broke
wrappers which compare exact modtimes, such as the hasher backend's
fingerprint and the VFS cache. Truncate the modtime to 100 ns
precision to match what the server stores.

backend/azurefiles/azurefiles.go

@@ -58,6 +58,9 @@ const (
 	maxFileSize           = 4 * fs.Tebi
 	defaultChunkSize      = 4 * fs.Mebi
 	storageDefaultBaseURL = "file.core.windows.net"
+	// smbTimePrecision is the precision the server stores
+	// modtimes with (100 ns FILETIME ticks)
+	smbTimePrecision = 100 * time.Nanosecond
 )
 
 func init() {
@@ -598,7 +601,10 @@ func (o *Object) SetModTime(ctx context.Context, t time.Time) error {
 	if err != nil {
 		return fmt.Errorf("unable to set modTime: %w", err)
 	}
-	o.modTime = t
+	// Truncate to the precision the server stores the modtime with
+	// to keep the in-memory modtime identical to the one a fresh
+	// listing returns.
+	o.modTime = t.Truncate(smbTimePrecision)
 	return nil
 }
 
@@ -769,10 +775,12 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 		return fmt.Errorf("update: failed to set properties: %w", err)
 	}
 
-	// Make sure Object is in sync
+	// Make sure Object is in sync, truncating the modtime to the
+	// precision the server stores it with so it is identical to the
+	// one a fresh listing returns
 	o.size = size
 	o.md5 = md5Hash
-	o.modTime = modTime
+	o.modTime = modTime.Truncate(smbTimePrecision)
 	o.contentType = contentType
 	return nil
 }