selftests/filesystems/overlayfs: test idmapped overlay mounts

Add a selftest for idmapping the merged overlay mount itself.  It
applies an idmapping to a freshly created (still detached) overlay
mount with mount_setattr(MOUNT_ATTR_IDMAP) and checks that:

  - getattr reports ownership mapped through the mount idmap;
  - a file created through the idmapped mount is stored on the upper
    layer with the corresponding overlay-final id;
  - chown through the idmapped mount round-trips;
  - an nfs_export overlay can be idmapped and decodable file handles
    round-trip through it with correctly mapped ownership.

The layers live on a private tmpfs and are owned by the host id so that
they map to id 0 through the test idmapping, allowing the root caller
to operate on them.

Link: https://patch.msgid.link/20260615-work-idmapped-overlayfs-v1-9-7381632aa402@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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