dm log: move dirty region log code into separate module

Move the dirty region log code into a separate module so
other targets can share the code.

Signed-off-by: Heinz Mauelshagen <hjm@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/dm-raid1.c b/drivers/md/dm-raid1.c
index 64489e7..c4ce011 100644
--- a/drivers/md/dm-raid1.c
+++ b/drivers/md/dm-raid1.c
@@ -1862,15 +1862,9 @@
 {
 	int r;
 
-	r = dm_dirty_log_init();
-	if (r)
-		return r;
-
 	r = dm_register_target(&mirror_target);
-	if (r < 0) {
+	if (r < 0)
 		DMERR("Failed to register mirror target");
-		dm_dirty_log_exit();
-	}
 
 	return r;
 }
@@ -1882,8 +1876,6 @@
 	r = dm_unregister_target(&mirror_target);
 	if (r < 0)
 		DMERR("unregister failed %d", r);
-
-	dm_dirty_log_exit();
 }
 
 /* Module hooks */