MAINTAINERS: add reference to device-mapper's linux-dm.git tree

Signed-off-by: Mike Snitzer <snitzer@redhat.com>

MAINTAINERS

@@ -2637,6 +2637,7 @@ M:	dm-devel@redhat.com
 L:	dm-devel@redhat.com
 W:	http://sources.redhat.com/dm
 Q:	http://patchwork.kernel.org/project/dm-devel/list/
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git
 T:	quilt http://people.redhat.com/agk/patches/linux/editing/
 S:	Maintained
 F:	Documentation/device-mapper/