src.ctf.fs: introduce and use ctf_fs_msg_iter_data::UP

src.ctf.fs: introduce and use ctf_fs_msg_iter_data::UP

Introduce ctf_fs_msg_iter_data::UP and use it in ctf_fs_iterator_init
and ctf_fs_iterator_finalize.

Change-Id: I448cedc164d93f9ba26da5640578889b0fff580c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8279
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12317
Tested-by: jenkins <jenkins@lttng.org>