MFC 346398 (by thj):

MFC 346398 (by thj):

  Add stat counter for ipv6 atomic fragments

  Add a stat counter to track ipv6 atomic fragments. Atomic fragments can be
  generated in response to invalid path MTU values, but are also a potential
  attack vector and considered harmful (see RFC6946 and RFC8021).

  While here add tracking of the atomic fragment counter to netstat and systat.

This should fix failing CI tests merged from head as
Resported by:	lwhsu