devicemapper: Add helper functions to allow deferred device removal

A lot of time device mapper devices leak across mount namespace which docker
does not know about and when docker tries to deactivate/delete device,
operation fails as device is open in some mount namespace.

Create a mechanism where one can defer the device deactivation/deletion
so that docker operation does not fail and device automatically goes
away when last reference to it is dropped.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

devicemapper/devmapper_wrapper_no_deferred_remove.go

@@ -0,0 +1,10 @@
+// +build linux,libdm_no_deferred_remove
+
+package devicemapper
+
+const LibraryDeferredRemovalSupport = false
+
+func dmTaskDeferredRemoveFct(task *CDmTask) int {
+	// Error. Nobody should be calling it.
+	return -1
+}