fgraph: Add READ_ONCE() when accessing fgraph_array[]

In the Linux kernel, the following vulnerability has been resolved:

fgraph: Add READ_ONCE() when accessing fgraph_array[]

In __ftrace_return_to_handler(), a loop iterates over the fgraph_array[]

elements, which are fgraph_ops. The loop checks if an element is a

fgraph_stub to prevent using a fgraph_stub afterward.

However, if the compiler reloads fgraph_array[] after this check, it might

race with an update to fgraph_array[] that introduces a fgraph_stub. This

could result in the stub being processed, but the stub contains a null

"func_hash" field, leading to a NULL pointer dereference.

To ensure that the gops compared against the fgraph_stub matches the gops

processed later, add a READ_ONCE(). A similar patch appears in commit

63a8dfb ("function_graph: Add READ_ONCE() when accessing fgraph_array[]").