fixed empty netns bug

[cgchinmay]

What type of PR is this?
bug fix

Which issue does this PR fix:
#1939

What does this PR do / Why do we need it:
Empty NetNS check was causing IP leak in certain edge cases for non-sgp pods. So have to make this fix

Testing done on this change:

Validated cni host-networking tests
Manually deleted sandbox container (edge case scenario). On pod delete, saw the DEL request invoked for CNI with an empty NetNS as seen below and the IPs was freed

{"level":"debug","ts":"2022-03-29T21:33:33.640Z","caller":"routed-eni-cni-plugin/cni.go:295","msg":"Prev Result: &{0.4.0 [{Name:eni84953baf01d Mac: Sandbox:} {Name:eth0 Mac: Sandbox:/proc/23288/ns/net}] [{Version:4 Interface:0xc0001cc260 Address:{IP:192.168.21.198 Mask:ffffffff} Gateway:<nil>}] [] {[]  [] []}}\n"}
{"level":"info","ts":"2022-03-29T21:33:33.640Z","caller":"routed-eni-cni-plugin/cni.go:295","msg":"Received CNI del request: ContainerID(c139669cc9f13aa65d2a0932fbde93bb7138c1cbba57021b97093fe917625008) Netns() IfName(eth0) Args(IgnoreUnknown=1;K8S_POD_NAMESPACE=default;K8S_POD_NAME=samplepod-8488bb6b58-gh5mb;K8S_POD_INFRA_CONTAINER_ID=c139669cc9f13aa65d2a0932fbde93bb7138c1cbba57021b97093fe917625008) Path(/opt/cni/bin) argsStdinData({\"cniVersion\":\"0.4.0\",\"mtu\":\"9001\",\"name\":\"aws-cni\",\"pluginLogFile\":\"/var/log/aws-routed-eni/plugin.log\",\"pluginLogLevel\":\"DEBUG\",\"prevResult\":{\"cniVersion\":\"0.4.0\",\"interfaces\":[{\"name\":\"eni84953baf01d\"},{\"name\":\"eth0\",\"sandbox\":\"/proc/23288/ns/net\"}],\"ips\":[{\"version\":\"4\",\"interface\":1,\"address\":\"192.168.21.198/32\"}],\"dns\":{}},\"type\":\"aws-cni\",\"vethPrefix\":\"eni\"})"}
{"level":"info","ts":"2022-03-29T21:33:33.642Z","caller":"routed-eni-cni-plugin/cni.go:295","msg":"Received del network response from ipamd for pod samplepod-8488bb6b58-gh5mb namespace default sandbox c139669cc9f13aa65d2a0932fbde93bb7138c1cbba57021b97093fe917625008: Success:true  IPv4Addr:\"192.168.21.198\""}
{"level":"debug","ts":"2022-03-29T21:33:33.642Z","caller":"routed-eni-cni-plugin/cni.go:425","msg":"TeardownNS: addr 192.168.21.198/32, deviceNumber 0"}
{"level":"info","ts":"2022-03-29T21:33:33.642Z","caller":"driver/driver.go:532","msg":"Delete toContainer rule for 192.168.21.198/32 "}
{"level":"error","ts":"2022-03-29T21:33:33.642Z","caller":"driver/driver.go:532","msg":"delete NS network: failed to delete host route for 192.168.21.198/32, no such process"}
{"level":"debug","ts":"2022-03-29T21:33:33.642Z","caller":"driver/driver.go:532","msg":"Tear down of NS complete"}

Automation added to e2e:

Not yet

Will this PR introduce any new dependencies?:

No

Will this break upgrades or downgrades. Has updating a running cluster been tested?:
No

Does this change require updates to the CNI daemonset config files to work?:

No

Does this PR introduce any user-facing change?:

No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[M00nF1sh]

/lgtm