src.ctf.fs: always set the correct trace IR stream ID

src.ctf.fs: always set the correct trace IR stream ID

In add_ds_file_to_ds_file_group(), we don't use the original CTF data
stream ID to set the trace IR stream ID whenever the beginning timestamp
is missing.

The comment says:

    if (begin_ns == -1) {
        /*
         * No beginning timestamp to sort the stream files
         * within a stream file group, so consider that this
         * file must be the only one within its group.
         */
        stream_instance_id.reset();
    }

This might be true, but there's no need to reset the data stream ID.

Then during data stream decoding, the trace IR stream ID always matches
the CTF IR data stream ID (if any), which enables some further checks.

Furthermore, a trace which goes from `src.ctf.fs` to `sink.ctf.fs` will
keep its original data stream IDs.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I3c6ea0a3cbd913017cd00c1c07c6c6bf83b8a534
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12771