gpfs: Fetch GPFS DOS attributes asynchronously

gpfs: Fetch GPFS DOS attributes asynchronously

This change introduces asynchronous handling of GPFS DOS attributes
using the pthreadpool infrastructure. This is part of the overall
effort to improve SMB directory listing performance in environments
with GPFS as the backend filesystem.

Signed-off-by: Pawan Sahu <Pawan.Sahu1@ibm.com>
Reviewed-by: Vinit Agnihotri <vagnihot@redhat.com>
Reviewed-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>
Autobuild-User(master): Günther Deschner <gd@samba.org>
Autobuild-Date(master): Tue Sep 30 13:08:16 UTC 2025 on atb-devel-224