Add a rate limit for ICMP6 informational messages and make the current
rate limit on ICMP6 error messages be MP safe.
To do this without increasing code duplication, move the bulk of the
badport_bandlim() function from the protocol-specific file ip_icmp.c
to the protocol-independent file subr_counter.c.
At the same time, break out the SCTP and TCP limits separately. This
removes the dependency on ip_icmp.c for an IPv6-only kernel.
(Note that the diff builds on changes from D10387.)
Release Notes: yes