Always use filepath.Join instead of path.Join

This patch cleans up pkg/util/mount/* and pkg/util/volume/* to always
use filepath.Join instead of path.Join. filepath.Join is preferred
because path.Join can have issues on Windows.

pkg/volume/util/device_util_linux.go

@@ -22,7 +22,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
-	"path"
+	"path/filepath"
 	"strconv"
 	"strings"
 
@@ -77,10 +77,10 @@ func (handler *deviceHandler) FindSlaveDevicesOnMultipath(dm string) []string {
 		return devices
 	}
 	disk := parts[2]
-	slavesPath := path.Join("/sys/block/", disk, "/slaves/")
+	slavesPath := filepath.Join("/sys/block/", disk, "/slaves/")
 	if files, err := io.ReadDir(slavesPath); err == nil {
 		for _, f := range files {
-			devices = append(devices, path.Join("/dev/", f.Name()))
+			devices = append(devices, filepath.Join("/dev/", f.Name()))
 		}
 	}
 	return devices