`ctf` plugin: `bt_bfcr`: use object's log level

As of this patch, the only user of `bt_bfcr` is `bt_msg_iter`, which
sets the new log level parameter to its own (global) log level.

The goal is to eventually have a per-component log level for the `ctf`
plugin's component classes.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ie77969d587c03c7d0257c19bb151ee22707b9842
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1487
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>