udf_cksum
crc = udf_cksum(((uint8_t *) tag) + UDF_DESC_TAG_LENGTH, crc_len);
crc = udf_cksum(((uint8_t *) tag) + UDF_DESC_TAG_LENGTH,
unsigned short udf_cksum(unsigned char *, int);
crc = udf_cksum(((uint8_t *) tag) + UDF_DESC_TAG_LENGTH, crc_len);
crc = udf_cksum(btag + UDF_DESC_TAG_LENGTH, crc_len);