Pad the icmp6stat structure so that we can add more counters in the
future without breaking compatibility again, last done in r358620.
Annotate the rarely executed error paths with __predict_false while
here.
Reviewed by: bz, melifaro
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D26578