fgraph: Use guard(mutex)(&ftrace_lock) for unregister_ftrace_graph()

fgraph: Use guard(mutex)(&ftrace_lock) for unregister_ftrace_graph()

The ftrace_lock is held throughout unregister_ftrace_graph(), use a guard
to simplify the error paths.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/20241028071307.770550792@goodmis.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>