fine-grained PI control

[Christoph Hellwig]

On Mon, Sep 23, 2024 at 09:59:10PM -0400, Martin K. Petersen wrote:
> Originally we had code in Linux allowing filesystems to attach
> additional metadata to bios which would then be stored in the app tag
> space. This was intended for backpointers but never really took off.
> However, if we move the burden of PI generation to filesystems as
> Christoph suggested, then we can revisit using the app tag space.

Just to make clear I want file systems to be able to opt into
generating PI, not get rid of the auto generation.  That is actually
kinda possible already, I just want to make it nicer.