Ignore error on enabling TCP early demux for old kernels

[mogren]

What type of PR is this?
bug

Which issue does this PR fix:
#1241

What does this PR do / Why do we need it:
For old kernels that do not have the tcp_early_demux kernel flag, the init container throws an error:

[centos@ip-192-168-9-194 ~]$ sysctl -w "net.ipv4.tcp_early_demux=1"
sysctl: cannot stat /proc/sys/net/ipv4/tcp_early_demux: No such file or directory

We need to ignore this warning:

[centos@ip-192-168-9-194 ~]$ sysctl -e -w "net.ipv4.tcp_early_demux=1"

From https://linux.die.net/man/8/sysctl:
-e Use this option to ignore errors about unknown keys.

The "early demux" feature was added in kernel 3.6, but the flag to disable it was not added until 4.6. This means that TCP health checks for pods using per-pod security groups will not work until at least kernel 4.6. UDP or exec checks should still work for pods with ENIs.

If an issue # is not available please add repro steps and logs from IPAMD/CNI showing the issue:
Start a CentOS image, run sysctl -w "net.ipv4.tcp_early_demux=1"

Testing done on this change:
See above

Automation added to e2e:
None

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.

[jayanthvn]

https://linux.die.net/man/8/sysctl (-e)option

Looks good :)

[jayanthvn]

I was discussing with Claes, should we instead throw a warning something like - "Current kernel version doesn't support disabling tcp v4 early demux and the issue #1212 will exists".