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/posix.go

@@ -610,7 +610,7 @@ func (s *posix) ReadFile(volume, path string, offset int64, buffer []byte, verif
 		return 0, errIsNotRegular
 	}
 
-	if verifier != nil && !verifier.IsVerified() {
+	if verifier != nil {
 		bufp := s.pool.Get().(*[]byte)
 		defer s.pool.Put(bufp)