ftrace: Assign RCU list variable with rcu_assign_ptr()

Use rcu_assign_ptr() to assign the list pointer as it is marked as RCU,
and this quiets the sparse warning:

   kernel/trace/ftrace.c:313:23: warning: incorrect type in assignment (different address spaces)
   kernel/trace/ftrace.c:313:23:    expected struct ftrace_ops [noderef] __rcu *
   kernel/trace/ftrace.c:313:23:    got struct ftrace_ops *

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Andrew Morton <[email protected]>
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

kernel/trace/ftrace.c

@@ -311,7 +311,7 @@ static int remove_ftrace_ops(struct ftrace_ops __rcu **list,
 			lockdep_is_held(&ftrace_lock)) == ops &&
 	    rcu_dereference_protected(ops->next,
 			lockdep_is_held(&ftrace_lock)) == &ftrace_list_end) {
-		*list = &ftrace_list_end;
+		rcu_assign_pointer(*list, &ftrace_list_end);
 		return 0;
 	}