icmp: use per rate limit randomized jitter

[tuexen]

Summary

Using the same random jitter for multiple rate limits allows an attacker to use one rate limiter to figure out the current jitter and then use this knowledge to de-randomize the other rate limiters.
This can be mitigated by using a separate randomized jitter for each rate limiter.
This issue was reported in Keyu Man et al.: SCAD: Towards a Universal and Automated Network Side-Channel Vulnerability Detection

[glebius]

sys/netinet/ip_icmp.c

1113

Let's

MPASS(which < BANDLIM_MAX);

Or copy a more verbose assertion from badport_bandlim().

sys/netinet6/icmp6.c

2782

Maybe same assertion here.

[tuexen]

Add KASSERT()s, as suggested by glebius@.

[tuexen]

sys/netinet/ip_icmp.c

1113

Done.

[tuexen]

sys/netinet6/icmp6.c

2782

Done.