fix(cgroups.plugin): do not add network devices if cgroup proc is in the host net ns

[ilyam8]

Summary

detect_veth_interfaces() works correctly only when the cgroup proc (cgroup.procs) is not in the host network namespace.

The bug was discovered when we installed Netdata on a Proxmox host with several VMs.

$ readlink /proc/self/ns/net
net:[4026532008]
$ nsenter -t $(cat /sys/fs/cgroup/qemu.slice/103.scope/cgroup.procs) -n readlink /proc/self/ns/net
net:[4026532008]
$ ps -o comm= --pid $(cat /sys/fs/cgroup/qemu.slice/103.scope/cgroup.procs)
kvm

The end result is adding all double-linked host interfaces to the last discovered VM cgroup.

Screenshot

Screenshot when the patch applied

Test Plan

Check VM cgroups network interfaces.

Additional Information

For users: How does this change affect me?