fgraph: Keep track of when fgraph_ops are registered or not

Add a warning if unregister_ftrace_graph() is called without ever
registering it, or if register_ftrace_graph() is called twice. This can
detect errors when they happen and not later when there's a side effect:

Link: https://lore.kernel.org/all/20250617120830.24fbdd62@gandalf.local.home/

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/20250701194451.22e34724@gandalf.local.home
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

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