geom_disk: Export attachment if present in XML config

[jrtc27]

Summary

This allows geom(8) to display the attachment such that an unprivileged
user can link up geom disks with their newbus counterparts, just as
diskinfo(8) can already do today with a device ioctl.

MFC after: 1 month

[jrtc27]

sys/geom/geom_disk.c

638

Hm, we don't actually need this to be overridable, it's always going to be a static thing drivers can just set to the right value. Will simplify by always using d_attachment, which is also simplifying for another change I'd like to make.

[jrtc27]

Don't bother with d_getattr call

[mav]

It seems BIO_GETATTR allows GEOM::attachment override. Though I am not saying it is very useful.

[jrtc27]

In D46468#1059227, @mav wrote:

It seems BIO_GETATTR allows GEOM::attachment override. Though I am not saying it is very useful.

I think it's more to allow BIO_GETATTRY as a way for userspace to query it. GEOM::fw{head,sector}s are similarly handled like that for BIO_GETATTR yet we don't consider d_getattr overriding them. GEOM::ident is a bit of a special case to allow it to be dynamic even though there's a d_ident (as per your rGccba710262443cf4cf2350f173750cf51572d3b9).

[imp]

There's a lot of other attributes that would be useful to export... but this is a good start.

I lean towards direct access here because geom_disk is allowed to know about itself...