src.ctf.fs: make ctf_fs_ds_file_group_create return a ctf_fs_ds_file_group::UP

src.ctf.fs: make ctf_fs_ds_file_group_create return a ctf_fs_ds_file_group::UP

Introduced ctf_fs_ds_file_group::UP, a unique_ptr type with a deleter
that calls ctf_fs_ds_file_group_destroy.  Change
ctf_fs_ds_file_group_create to return that type.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Ic89ec8edc5d7dd779c836beb962dcad7e45ab996
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8245
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12284