gluster: the glfs_io_cbk callback function pointer adds pre/post stat args

gluster: the glfs_io_cbk callback function pointer adds pre/post stat args

The glfs_*_async() functions do a callback once finished. This callback
has changed its arguments, pre- and post-stat structures have been
added. This makes it possible to improve caching, which is useful for
Samba and NFS-Ganesha, but not so much for QEMU. Gluster 6 is the first
release that includes these new arguments.

With an additional detection in ./configure, the new arguments can
conditionally get included in the glfs_io_cbk handler.

Signed-off-by: Niels de Vos <ndevos@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>