On IPv6, tcptracer ports always appears as zeros:

Tracing TCP established connections. Ctrl-C to end.
T  PID    COMM             IP SADDR            DADDR            SPORT  DPORT
X  7055   nc               4  127.0.0.1        127.0.0.1        49476  9999
C  7074   nc               4  127.0.0.1        127.0.0.1        49478  9999
X  7074   nc               4  127.0.0.1        127.0.0.1        49478  9999
C  7085   nc               6  [::]             [0:0:0:1::]      0      0
X  7085   nc               6  [::]             [0:0:0:1::]      0      0
C  7086   nc               6  [::]             [0:0:0:1::]      0      0

This seems related to alignment issue wrt to the __int128 type in
tcp_ipv6_event_t structure. Moving the u8 field ip to the end of the
structure fixes the issue.

Fixes #2781