function_graph: Restore direct mode when callbacks drop to one

When registering a second fgraph callback, direct path is disabled and
array loop is used instead.  When ftrace_graph_active falls back to one,
we try to re-enable direct mode via ftrace_graph_enable_direct(true, ...).
But ftrace_graph_enable_direct() incorrectly disables the static key
rather than enabling it.  This leaves fgraph_do_direct permanently off
after first multi-callback transition, so direct fast mode is never
restored.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260213142932519cuWSpEXeS4-UnCvNXnK2P@zte.com.cn
Fixes: cc60ee8 ("function_graph: Use static_call and branch to optimize entry function")
Signed-off-by: Shengming Hu <hu.shengming@zte.com.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: HSM6236

kernel/trace/fgraph.c

@@ -1303,7 +1303,7 @@ static void ftrace_graph_enable_direct(bool enable_branch, struct fgraph_ops *go
 	static_call_update(fgraph_func, func);
 	static_call_update(fgraph_retfunc, retfunc);
 	if (enable_branch)
-		static_branch_disable(&fgraph_do_direct);
+		static_branch_enable(&fgraph_do_direct);
 }
 
 static void ftrace_graph_disable_direct(bool disable_branch)