generate: Adjust to Spec.Linux being a pointer

Catch up with opencontainers/runtime-spec@6323157 (specs-go/config:
Make Linux and Solaris omitempty (again), 2016-06-17, #502).

The "does the marshaled JSON look like '{}'?" check is a pretty cheap
trick, but it was the easiest way I could think of for "is there
anything useful in here?".  An alternative approach that conditionally
added an &rspec.Linux{} only if needed seemed too tedious.

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

generate.go

@@ -142,7 +142,12 @@ func modify(spec *rspec.Spec, context *cli.Context) error {
 		spec.Process.Args = make([]string, 0)
 	}
 	spec.Process.SelinuxLabel = context.String("selinux-label")
-	spec.Linux.CgroupsPath = sPtr(context.String("cgroups-path"))
+	if spec.Linux == nil {
+		spec.Linux = &rspec.Linux{}
+	}
+	if context.IsSet("cgroups-path") {
+		spec.Linux.CgroupsPath = sPtr(context.String("cgroups-path"))
+	}
 	spec.Linux.MountLabel = context.String("mount-label")
 	spec.Platform.OS = context.String("os")
 	spec.Platform.Arch = context.String("arch")
@@ -228,6 +233,14 @@ func modify(spec *rspec.Spec, context *cli.Context) error {
 		return err
 	}
 
+	buf, err := json.Marshal(spec.Linux)
+	if err != nil {
+		return err
+	}
+	if string(buf) == "{}" {
+		spec.Linux = nil
+	}
+
 	return nil
 }
 
@@ -807,7 +820,7 @@ func getDefaultTemplate() *rspec.Spec {
 				Options:     []string{"nosuid", "noexec", "nodev", "ro"},
 			},
 		},
-		Linux: rspec.Linux{
+		Linux: &rspec.Linux{
 			Resources: &rspec.Resources{
 				Devices: []rspec.DeviceCgroup{
 					{