blk-wbt: account any writing command as a write

We currently special case WRITE and FLUSH, but we should really
just include any command with the write bit set. This ensures
that we account DISCARD.
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-wbt.c

@@ -701,7 +701,7 @@ static int wbt_data_dir(const struct request *rq)
 
 	if (op == REQ_OP_READ)
 		return READ;
-	else if (op == REQ_OP_WRITE || op == REQ_OP_FLUSH)
+	else if (op_is_write(op))
 		return WRITE;
 
 	/* don't account */