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.go

@@ -112,6 +112,7 @@ var (
 	DmUdevGetSyncSupport   = dmUdevGetSyncSupportFct
 	DmCookieSupported      = dmCookieSupportedFct
 	LogWithErrnoInit       = logWithErrnoInitFct
+	DmTaskDeferredRemove   = dmTaskDeferredRemoveFct
 )
 
 func free(p *C.char) {