Block: Fix block/elevator.c elevator_get() off-by-one error

elevator_get() not check the name length, if the name length > sizeof(elv),
elv will miss the '\0'. And elv buffer will be replace "-iosched" as something
like aaaaaaaaa, then call request_module() can load an not trust module.
Signed-off-by: Zhitong Wang <zhitong.wangzt@alibaba-inc.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/elevator.c

@@ -154,7 +154,7 @@ static struct elevator_type *elevator_get(const char *name)
 
 		spin_unlock(&elv_list_lock);
 
-		sprintf(elv, "%s-iosched", name);
+		snprintf(elv, sizeof(elv), "%s-iosched", name);
 
 		request_module("%s", elv);
 		spin_lock(&elv_list_lock);