tcp: improve initializing the fields in tcp_log_buffer

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[].

Reviewed by: rrs, Peter Lei
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D49652