XL/Erasure: Make bit-rot verification based on xl.json algo. (#2299)

Currently `xl.json` saves algorithm information for bit-rot
verification. Since the bit-rot algo's can change in the
future make sure the erasureReadFile doesn't default to
a particular algo. Instead use the checkSumInfo.

erasure-utils_test.go

@@ -23,8 +23,7 @@ import (
 // Test validates the number hash writers returned.
 func TestNewHashWriters(t *testing.T) {
 	diskNum := 8
-	hashWriters := newHashWriters(diskNum)
-
+	hashWriters := newHashWriters(diskNum, bitRotAlgo)
 	if len(hashWriters) != diskNum {
 		t.Errorf("Expected %d hashWriters, but instead got %d", diskNum, len(hashWriters))
 	}