lib: graph API: return borrowed references when adding to an object

Before this patch, the following functions return a new reference when
their last parameter is not `NULL`:

* bt_graph_add_filter_component()
* bt_graph_add_filter_component_with_initialize_method_data()
* bt_graph_add_simple_sink_component()
* bt_graph_add_sink_component()
* bt_graph_add_sink_component_with_initialize_method_data()
* bt_graph_add_source_component()
* bt_graph_add_source_component_with_initialize_method_data()
* bt_graph_connect_ports()
* bt_self_component_filter_add_input_port()
* bt_self_component_filter_add_output_port()
* bt_self_component_sink_add_input_port()
* bt_self_component_source_add_output_port()

I'm changing this so that they return a borrowed reference instead. This
is more in line with other non-creating functions which always return
borrowed references.

It's okay to borrow here because the object to which you add an object
becomes its owner anyway.

Most sites are updated by removing the *_put_ref() call as the reference
is now borrowed.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I71a5e18760504d8f8610162e3f6d7bd8d87474f9
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2762
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>