fgraph: Fix missing unlock in register_ftrace_graph()

Use guard(mutex)() to acquire and automatically release ftrace_lock,
fixing the issue of not unlocking when calling cpuhp_setup_state()
fails.

Fixes smatch warning:

kernel/trace/fgraph.c:1317 register_ftrace_graph() warn: inconsistent returns '&ftrace_lock'.

Link: https://lore.kernel.org/[e-mail]
Fixes: 2c02f7375e65 ("fgraph: Use CPU hotplug mechanism to initialize idle shadow stacks")
Reported-by: kernel test robot <[e-mail]>
Reported-by: Dan Carpenter <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]/
Suggested-by: Steven Rostedt <[e-mail]>
Signed-off-by: Li Huafei <[e-mail]>
Acked-by: Masami Hiramatsu (Google) <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

kernel/trace/fgraph.c

···
 	int ret = 0;
 	int i = -1;
 
-	mutex_lock(&ftrace_lock);
+	guard(mutex)(&ftrace_lock);
 
 	if (!fgraph_initialized) {
 		ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "fgraph_idle_init",
···
 	}
 
 	i = fgraph_lru_alloc_index();
-	if (i < 0 || WARN_ON_ONCE(fgraph_array[i] != &fgraph_stub)) {
-		ret = -ENOSPC;
-		goto out;
-	}
+	if (i < 0 || WARN_ON_ONCE(fgraph_array[i] != &fgraph_stub))
+		return -ENOSPC;
 	gops->idx = i;
 
 	ftrace_graph_active++;
···
 		gops->saved_func = NULL;
 		fgraph_lru_release_index(i);
 	}
-out:
-	mutex_unlock(&ftrace_lock);
 	return ret;
 }