src.ctf.fs: switch to using ctf_fs_trace::UP where possible

src.ctf.fs: switch to using ctf_fs_trace::UP where possible

Change a few function in fs.cpp to use ctf_fs_trace::UP.  Change a use of
GPtrArray to an std::vector.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I7bd8fdefea538cc778bbca5157b92e960052e1cc
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8242
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12281