libtracefs: Free buf in clear_func_filter()

The buf variable was allocated via tracefs_instance_file_read() but must be
freed with free().

Link: https://lore.kernel.org/linux-trace-devel/20231228215433.54854-5-rostedt@goodmis.org

Fixes: 789e82d7 ("libtracefs: New API to reset ftrace instance")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>