libtracefs: Add tracefs_instance_get/set_buffer_percent()

Add an API that allows modifying the buffer_percent of the tracing ring
buffer. That is the amount that is written into the ring buffer before the
trace is woken up.

Link: https://lore.kernel.org/linux-trace-devel/20231228215433.54854-13-rostedt@goodmis.org

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>