vfs gd: ignore quota setup errors

This is a fix to regression in vfs graph driver introduced by
commit 7a1618ced359a3ac92 ("add quota support to VFS graphdriver").

On some filesystems, vfs fails to init with the following error:

> Error starting daemon: error initializing graphdriver: Failed to mknod
> /go/src/github.com/docker/docker/bundles/test-integration/d6bcf6de610e9/root/vfs/backingFsBlockDev:
> function not implemented

As quota is not essential for vfs, let's ignore (but log as a warning) any error
from quota init.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

daemon/graphdriver/vfs/driver.go

@@ -35,9 +35,7 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		return nil, err
 	}
 
-	if err := setupDriverQuota(d); err != nil {
-		return nil, err
-	}
+	setupDriverQuota(d)
 
 	return graphdriver.NewNaiveDiffDriver(d, uidMaps, gidMaps), nil
 }

daemon/graphdriver/vfs/quota_linux.go

@@ -2,20 +2,21 @@
 
 package vfs
 
-import "github.com/docker/docker/daemon/graphdriver/quota"
+import (
+	"github.com/docker/docker/daemon/graphdriver/quota"
+	"github.com/sirupsen/logrus"
+)
 
 type driverQuota struct {
 	quotaCtl *quota.Control
 }
 
-func setupDriverQuota(driver *Driver) error {
+func setupDriverQuota(driver *Driver) {
 	if quotaCtl, err := quota.NewControl(driver.home); err == nil {
 		driver.quotaCtl = quotaCtl
 	} else if err != quota.ErrQuotaNotSupported {
-		return err
+		logrus.Warnf("Unable to setup quota: %v\n", err)
 	}
-
-	return nil
 }
 
 func (d *Driver) setupQuota(dir string, size uint64) error {