dm thin: correct module description

Remove duplicate copy of string "device-mapper" (DM_NAME) from
MODULE_DESCRIPTION.
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>

drivers/md/dm-thin.c

@@ -2767,6 +2767,6 @@ static void dm_thin_exit(void)
 module_init(dm_thin_init);
 module_exit(dm_thin_exit);
 
-MODULE_DESCRIPTION(DM_NAME "device-mapper thin provisioning target");
+MODULE_DESCRIPTION(DM_NAME " thin provisioning target");
 MODULE_AUTHOR("Joe Thornber <dm-devel@redhat.com>");
 MODULE_LICENSE("GPL");