blk-wbt: don't throttle discard or write zeroes

Both of these are metadata only commands that are not issued by the
writeback code and not directly relevant to the writeback bandwith.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@fb.com>

block/blk-wbt.c

@@ -575,10 +575,9 @@ static inline bool wbt_should_throttle(struct rq_wb *rwb, struct bio *bio)
 	const int op = bio_op(bio);
 
 	/*
-	 * If not a WRITE (or a discard or write zeroes), do nothing
+	 * If not a WRITE, do nothing
 	 */
-	if (!(op == REQ_OP_WRITE || op == REQ_OP_DISCARD ||
-				op == REQ_OP_WRITE_ZEROES))
+	if (op != REQ_OP_WRITE)
 		return false;
 
 	/*