projectquota: treat ENOSYS as quota unsupported

If mknod() returns ENOSYS, it most probably means quota is not supported
here, so return the appropriate error.

This is a conservative* 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

Reported-by: Brian Goff <cpuguy83@gmail.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

daemon/graphdriver/quota/projectquota.go

@@ -350,11 +350,17 @@ func makeBackingFsDev(home string) (string, error) {
 	backingFsBlockDev := path.Join(home, "backingFsBlockDev")
 	// Re-create just in case someone copied the home directory over to a new device
 	unix.Unlink(backingFsBlockDev)
-	if err := unix.Mknod(backingFsBlockDev, unix.S_IFBLK|0600, int(stat.Dev)); err != nil {
+	err := unix.Mknod(backingFsBlockDev, unix.S_IFBLK|0600, int(stat.Dev))
+	switch err {
+	case nil:
+		return backingFsBlockDev, nil
+
+	case unix.ENOSYS:
+		return "", ErrQuotaNotSupported
+
+	default:
 		return "", fmt.Errorf("Failed to mknod %s: %v", backingFsBlockDev, err)
 	}
-
-	return backingFsBlockDev, nil
 }
 
 func hasQuotaSupport(backingFsBlockDev string) (bool, error) {