tcp: improve initializing the fields in tcp_log_buffer

[tuexen]

Summary

Initialize the fields in the tcp_log_buffer in the sequence they appear in the structure and add the initialization of tlb_flex1, tlb_flex2, and _pad[].

[glebius]

IMHO, these macros are more obfuscation than a helper. I would write one big sparse initializer here and let compiler optimize wrt padding zeroing.

[rrs]

Note I do agree with Gleb here, these macros are just nasty and not very clear. However I think that making such a change should be a separate commit from this one!