kube-proxy: only set route_localnet if required

[aojea]

kube-proxy sets the sysctl net.ipv4.conf.all.route_localnet=1
so NodePort services can be accessed on the loopback addresses in
IPv4, but this may present security issues.

Leverage the --nodeport-addresses flag to opt-out of this feature,
if the list is not empty and none of the IP ranges contains a
loopback address this sysctl is not set.

In addition, add a warning to inform users about this behavior.

/kind cleanup

kube-proxy doesn't set the sysctl net.ipv4.conf.all.route_localnet=1 if no IPv4 loopback address is selected by the nodePortAddresses configuration parameter.

Fixes: #106976

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[aojea]

/assign @thockin

[dcbw]

This LGTM but I'd like a second opinion.

[dims]

cc @jayunit100

[thockin]

I like the idea.

Should we also add a flag to disable localhost node-ports?

[cyclinder]

/lgtm
I think we should also add a flag for it(default true). When set it to false, we don't need set sysctl net.ipv4.conf.all.route_localnet=1.
When this pr merged, I can take this one.

[thockin]

Minor thing :)

/lgtm
/approve

[thockin]

Thanks!

/lgtm
/approve

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