libtracefs: Have tracefs_instance_tracers() return instance tracers

The tracefs_instance_tracers() was always returning the top level tracers
and not the tracers that were available for the given instance.

Link: https://lore.kernel.org/20240820173604.29693def@gandalf.local.home
Fixes: 185019c0 ("libtracefs: Add tracefs_instance_tracers() API")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>