libcontainer/specconv/spec_linux: Support empty 'type' for bind mounts

From the "Creating a bind mount" section of mount(2) [1]:

> If mountflags includes MS_BIND (available since Linux 2.4), then
> perform a bind mount...
>
> The filesystemtype and data arguments are ignored.

This commit adds support for configurations that leave the OPTIONAL
type [2] unset for bind mounts.  There's a related spec-example change
in flight with [3], although my personal preference would be a more
explicit spec for the whole mount structure [4].

[1]: http://man7.org/linux/man-pages/man2/mount.2.html
[2]: https://github.com/opencontainers/runtime-spec/blame/v1.0.1/config.md#L102
[3]: https://github.com/opencontainers/runtime-spec/pull/954
[4]: https://github.com/opencontainers/runtime-spec/pull/771

Signed-off-by: W. Trevor King <wking@tremily.us>

libcontainer/specconv/spec_linux.go

@@ -269,13 +269,17 @@ func CreateLibcontainerConfig(opts *CreateOpts) (*configs.Config, error) {
 func createLibcontainerMount(cwd string, m specs.Mount) *configs.Mount {
 	flags, pgflags, data, ext := parseMountOptions(m.Options)
 	source := m.Source
-	if m.Type == "bind" {
+	device := m.Type
+	if flags|unix.MS_BIND != 0 {
+		if device == "" {
+			device = "bind"
+		}
 		if !filepath.IsAbs(source) {
 			source = filepath.Join(cwd, m.Source)
 		}
 	}
 	return &configs.Mount{
-		Device:           m.Type,
+		Device:           device,
 		Source:           source,
 		Destination:      m.Destination,
 		Data:             data,