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_deferred_remove.go

@@ -0,0 +1,15 @@
+// +build linux,!libdm_no_deferred_remove
+
+package devicemapper
+
+/*
+#cgo LDFLAGS: -L. -ldevmapper
+#include <libdevmapper.h>
+*/
+import "C"
+
+const LibraryDeferredRemovalSupport = true
+
+func dmTaskDeferredRemoveFct(task *CDmTask) int {
+	return int(C.dm_task_deferred_remove((*C.struct_dm_task)(task)))
+}