src.ctf.fs: make ctf_fs_make_port_name return a GCharUP

src.ctf.fs: make ctf_fs_make_port_name return a GCharUP

Make ctf_fs_make_port_name return a GCharUP, which then helps use
automatic memory management in the callers.

Change-Id: I9191ddbafa86a4788fc5e16fc561dfda7696cfa6
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8166
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12267
Tested-by: jenkins <jenkins@lttng.org>