src.ctf.fs: store index entry objects instead of pointers

src.ctf.fs: store index entry objects instead of pointers

Store `ctf_fs_ds_index_entry` objects in a flag vector, instead of a
vector of pointers.

Change-Id: I4fea078cddf97292cc1f040ca998acef465d6808
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8348
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12343
Tested-by: jenkins <jenkins@lttng.org>