LGTM

I've done the vendor import but keep getting tree conflicts when trying to merge it to HEAD

Thank you. Changes have been made. As soon as make universe completes I'll commit this.

kib, I miunderstood you at first, I think this is what you meant.
I also updated the comment.

Changes based on feedback from kib

I am not qualified to make the requested changes to this patch, but I know someone who is and I'll ad him to this review.

Is there a way to check that at run time or would it just be a mount time option? In our use case a filesystem might start small and grow over time.

The default value for this sysctl (30) is taken from upstream, but prior to this change the default is essentially 0 (this feature is disabled)

Run time testing shows this as working properly with no ill effects.

We've been using this in FreeNAS for a while, looks good.