icmp6: Count packets dropped due to an invalid hop limit

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