Given an offset of the file, checks if the corresponding block (by considering the file as 0-based continuous block ranges) is covered by the SortedRangeSet. If so, returns the offset within this SortedRangeSet. For example, the 4106-th byte of a file is from block 1, assuming a block size of 4096-byte. The mapped offset within a SortedRangeSet("1-9 15-19") is 10. An offset of 65546 falls into t