src.ctf.fs: remove ctf_fs_ds_index::UP

src.ctf.fs: remove ctf_fs_ds_index::UP

We don't need to dynamically allocate ctf_fs_ds_index objects, we can
store / return / pass objects directly.  Remove ctf_fs_ds_index::UP and
update all users.

Change-Id: I7ce9de83b111c782299a1caebea3ead0fd945cb5
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8401
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12344
Tested-by: jenkins <jenkins@lttng.org>