ftrace: Show subops in enabled_functions

The function graph infrastructure uses subops of the function tracer.
These are not shown in enabled_functions. Add a "subops:" section to the
enabled_functions line to show what functions are attached via subops. If
the subops is from the function_graph infrastructure, then show the entry
and return callbacks that are attached.

Here's an example of the output:

schedule_on_each_cpu (1)                tramp: 0xffffffffc03ef000 (ftrace_graph_func+0x0/0x60) ->ftrace_graph_func+0x0/0x60     subops: {ent:trace_graph_entry+0x0/0x20 ret:trace_graph_return+0x0/0x150}

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/20250410153830.5d97f108@gandalf.local.home
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

include/linux/ftrace.h

@@ -328,6 +328,7 @@ ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops);
  * DIRECT - Used by the direct ftrace_ops helper for direct functions
  *            (internal ftrace only, should not be used by others)
  * SUBOP  - Is controlled by another op in field managed.
+ * GRAPH  - Is a component of the fgraph_ops structure
  */
 enum {
 	FTRACE_OPS_FL_ENABLED			= BIT(0),
@@ -349,6 +350,7 @@ enum {
 	FTRACE_OPS_FL_PERMANENT                 = BIT(16),
 	FTRACE_OPS_FL_DIRECT			= BIT(17),
 	FTRACE_OPS_FL_SUBOP			= BIT(18),
+	FTRACE_OPS_FL_GRAPH			= BIT(19),
 };
 
 #ifndef CONFIG_DYNAMIC_FTRACE_WITH_ARGS