Skip further checks for quota in user namespaces

Commit 7a1618ced3 regresses running Docker
in user namespaces. The new check for whether quota are supported calls
NewControl() which in turn calls makeBackingFsDev() which tries to
mknod(). Skip quota tests when we detect that we are running in a user
namespace and return ErrQuotaNotSupported to the caller. This just
restores the status quo.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

daemon/graphdriver/quota/projectquota.go

@@ -58,6 +58,7 @@ import (
 	"path/filepath"
 	"unsafe"
 
+	rsystem "github.com/opencontainers/runc/libcontainer/system"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
 )
@@ -98,6 +99,14 @@ type Control struct {
 // project ids.
 //
 func NewControl(basePath string) (*Control, error) {
+	//
+	// If we are running in a user namespace quota won't be supported for
+	// now since makeBackingFsDev() will try to mknod().
+	//
+	if rsystem.RunningInUserNS() {
+		return nil, ErrQuotaNotSupported
+	}
+
 	//
 	// create backing filesystem device node
 	//