ipv6: icmp: remove obsolete code in icmpv6_xrlim_allow()

ipv6: icmp: remove obsolete code in icmpv6_xrlim_allow()

Following part was needed before the blamed commit, because
inet_getpeer_v6() second argument was the prefix.

	/* Give more bandwidth to wider prefixes. */
	if (rt->rt6i_dst.plen < 128)
		tmo >>= ((128 - rt->rt6i_dst.plen)>>5);

Now inet_getpeer_v6() retrieves hosts, we need to remove
@tmo adjustement or wider prefixes likes /24 allow 8x
more ICMP to be sent for a given ratelimit.

As we had this issue for a while, this patch changes net.ipv6.icmp.ratelimit
default value from 1000ms to 100ms to avoid potential regressions.

Also add a READ_ONCE() when reading net->ipv6.sysctl.icmpv6_time.

Fixes: fd0273d7939f ("ipv6: Remove external dependency on rt6i_dst and rt6i_src")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260216142832.3834174-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>