Adjust inputs order for the decode API in raw erasure coder

Currently we used the parity units + data units order for the inputs, erasedIndexes and outputs parameters in the decode call in raw erasure coder, which inherited from HDFS-RAID due to impact enforced by GaliosField. As Zhe Zhang pointed and Yi Liu felt, we'd better change the order to make it natural for HDFS usage, where usually data blocks are before parity blocks in a group. Doing this would avoid some reordering tricky logic.