src.ctf.fs: make ctf_fs_ds_file_create return a unique_ptr

src.ctf.fs: make ctf_fs_ds_file_create return a unique_ptr

Introduce ctf_fs_ds_file::UP, make ctf_fs_ds_file_create return it.

Change-Id: I0139e9504d65425bf53fb21a5b81a4e48da4fe4e
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8268
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12306
Tested-by: jenkins <jenkins@lttng.org>