Reduce verbosity of missing kernel support warning for secure symlink

[strideynet]

Closes #11755

Follows the same solution as used elsewhere, using a sync.Once to ensure warnings related to the missing syscall are only displayed once. This is a bit of a hacky solution, but matches the solution already in use, and only controls a log message.

[strideynet]

@timothyb89 - by re-using the same sync.Once here, the message will only be displayed once across creation and writing. I can't see any reason to display it once for creation and once for writing, but wanted to run this past you.

[timothyb89]

Looks fine to me, thanks!

[timothyb89]

Forgot to mention, we should backport this to branch/v9

[strideynet]

Hmm, we could ! I'm not too sure what our process is on backporting though, do we have a bot for this ?

[timothyb89]

I think we've got a bot in the works, but for now I'd just:

git checkout branch/b9
git checkout -b user/v9/branchname
git cherry-pick 9348ef172f86dc4c5f70a8f053f02fc54fb818e5

... and push up a PR for the result, with a base branch of branch/v9.

[rosstimothy]

Hmm, we could ! I'm not too sure what our process is on backporting though, do we have a bot for this ?

If you add labels to your PR before you merge, e.g. backport/branch/v9, the bot will automatically do the backports for you:
See #12373 for an example

[strideynet]

Thanks @rosstimothy, I'll keep that in mind next time.

@timothyb89 I'll raise that now :)