vfs_glusterfs: Fix a memory leak in AIO

Signed-off-by: Jose A. Rivera <jarrpa@samba.org>
Reviewed-by: Ira Cooper <ira@samba.org>

Autobuild-User(master): Ira Cooper <ira@samba.org>
Autobuild-Date(master): Thu Dec 17 20:55:22 CET 2015 on sn-devel-144
