s3-vfs: add glusterfs_fuse vfs module.

s3-vfs: add glusterfs_fuse vfs module.

This module only implements the get_real_filename function by accessing
a distinct extended attribute that is available over a glusterfs fuse
mount.

By implementing this vfs function users of a glusterfs fuse mount
achieve a much better performance in create based workloads where samba
then can avoid trying multiple case folding options to detect the real
filename.

Patch is based on an initial patch provided by
Poornima G <pgurusid@redhat.com>

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13774

Guenther

Signed-off-by: Günther Deschner <gd@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Günther Deschner <gd@samba.org>
Autobuild-Date(master): Tue Jan 22 18:37:56 CET 2019 on sn-devel-144

(cherry picked from commit adffe0dcf002aa4721dc7897261895e3486d5271)