blk-wbt: Use tracepoint_string() for wbt_step tracepoint string literals

Use tracepoint_string() for string literals that are used in the
wbt_step tracepoint, so that userspace tools can display the string
content.
Signed-off-by: NTommi Rantala <tommi.t.rantala@nokia.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-wbt.c

@@ -313,7 +313,7 @@ static void scale_up(struct rq_wb *rwb)
 	calc_wb_limits(rwb);
 	rwb->unknown_cnt = 0;
 	rwb_wake_all(rwb);
-	rwb_trace_step(rwb, "scale up");
+	rwb_trace_step(rwb, tracepoint_string("scale up"));
 }
 
 static void scale_down(struct rq_wb *rwb, bool hard_throttle)
@@ -322,7 +322,7 @@ static void scale_down(struct rq_wb *rwb, bool hard_throttle)
 		return;
 	calc_wb_limits(rwb);
 	rwb->unknown_cnt = 0;
-	rwb_trace_step(rwb, "scale down");
+	rwb_trace_step(rwb, tracepoint_string("scale down"));
 }
 
 static void rwb_arm_timer(struct rq_wb *rwb)