statx: Add an example of using this to determine mountpoint

[cgwalters]

• This use case is a specific reason to directly use the statx
  system call
• It's a bit of a subtle API because there's 4 different flags
  involved, and it's not necessarily obvious that the relevant
  bitmasks aren't (yet) exposed in rustix, so one needs to use
  libc/linux-raw-sys directly
• On the general principle of adding examples

[sunfishcode]

STATX_ATTR_MOUNT_ROOT is an stx_attributues_mask flag, while the StatxFlags type is only for the flags argument to the statx call.

Rustix currently just returns the stx_attributes_mask mask as the raw u64 value, so in the code where you want to use this, you can test it for libc::STAX_ATTR_MOUNT_ROOT manually.

Another option would be to change rustix to always use a custom statx struct so that it can replace stx_attributes_mask with a bitflags type with all the STATX_ATTR_* flags.

[cgwalters]

Sorry for sending a broken PR here, and thanks for the review. I will try to salvage this by updating this PR to instead add an example.

I've pushed an updated commit but at least here the GH user interface hasn't updated? (Ah yes status is degraded).

[sunfishcode]

Looks good, thanks!

[cgwalters]

Another option would be to change rustix to always use a custom statx struct so that it can replace stx_attributes_mask with a bitflags type with all the STATX_ATTR_* flags.

Indeed, this seems like a good followup. May take a look at that at some point, but at least hopefully for now the example leads people in the right direction.