add checksum for both pad-1 and pad-tail

[sh4run]

Tests show GFW may change one or several bytes in a packet then replay it to check the response of its victims. At this moment the pad data is simply skipped at the receiving side. Change of pad data cannot be detected and the packet is passed for further process after header decryption. It is not a serious problem though as the packet very likely is dropped in timestamp check.

The checksum fields are to detect any tampering of pad data.