macros: ICMPSTAT_ADD(), ICMPSTAT_INC(), ICMP6STAT_ADD(), and
ICMP6STAT_INC(), rather than directly manipulating the fields
of these structures across the kernel. This will make it
easier to change the implementation of these statistics,
such as using per-CPU versions of the data structures.
In on case, icmp6stat members are manipulated indirectly, by
icmp6_errcount(), and this will require further work to fix
for per-CPU stats.
MFC after: 3 days
4801e9aee9