Return bit-rot verified data instead of re-reading from disk (#5568)

- Data from disk was being read after bitrot verification to return
  data for GetObject. Strictly speaking this does not guarantee bitrot
  protection, as disks may return bad data even temporarily.

- This fix reads data from disk, verifies data for bitrot and then
  returns data to the client directly.

cmd/utils.go

@@ -294,3 +294,23 @@ func jsonSave(f interface {
 	}
 	return nil
 }
+
+// ceilFrac takes a numerator and denominator representing a fraction
+// and returns its ceiling. If denominator is 0, it returns 0 instead
+// of crashing.
+func ceilFrac(numerator, denominator int64) (ceil int64) {
+	if denominator == 0 {
+		// do nothing on invalid input
+		return
+	}
+	// Make denominator positive
+	if denominator < 0 {
+		numerator = -numerator
+		denominator = -denominator
+	}
+	ceil = numerator / denominator
+	if numerator > 0 && numerator%denominator != 0 {
+		ceil++
+	}
+	return
+}