icmp: improve INVARIANTS check

[tuexen]

Summary

Actually check the conditions, which are enforced by the error checking code instead of a condition which is

• checking a number to be non-negative instead of positive
• depending on a random number

[cc]

Additional comment:

% sysctl -d net.inet.icmp.icmplim_jitter
net.inet.icmp.icmplim_jitter: Random icmplim jitter adjustment limit

May document this net.inet.icmp.icmplim_jitter in ICMP(4).

[glebius]

Thanks! Please apply same changes to netinet6/icmp6.c

[tuexen]

Consistently handle ICMPv4 and ICMPv6 as suggested by Gleb.

[tuexen]

In D48001#1094533, @glebius wrote:

Thanks! Please apply same changes to netinet6/icmp6.c

Done. Thanks.

[tuexen]

In D48001#1094461, @cc wrote:

Additional comment:

% sysctl -d net.inet.icmp.icmplim_jitter
net.inet.icmp.icmplim_jitter: Random icmplim jitter adjustment limit

May document this net.inet.icmp.icmplim_jitter in ICMP(4).

Improved man page in D48025.