src.ctf.fs: initialize ctf_fs_ds_index_entry::packet_seq_num to UINT64_MAX in class

src.ctf.fs: initialize ctf_fs_ds_index_entry::packet_seq_num to UINT64_MAX in class

Initialize the field directly to the right value.  The field was
originally initialized to 0 to mimic the g_new0 behavior.

Change-Id: Ia1df1be730b77ca0d69ae05dae12bd60ef9e9ed8
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8346
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>