ftrace: Look up the address of return_to_handler() using helpers

This ensures that we use the right address on architectures that use
function descriptors.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/8f6f14d192a994008ac370ce14036bbe67224c7d.1567707399.git.naveen.n.rao@linux.vnet.ibm.com

kernel/trace/fgraph.c

@@ -276,7 +276,7 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
 	int index = task->curr_ret_stack;
 	int i;
 
-	if (ret != (unsigned long)return_to_handler)
+	if (ret != (unsigned long)dereference_kernel_function_descriptor(return_to_handler))
 		return ret;
 
 	if (index < 0)
@@ -294,7 +294,7 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
 {
 	int task_idx;
 
-	if (ret != (unsigned long)return_to_handler)
+	if (ret != (unsigned long)dereference_kernel_function_descriptor(return_to_handler))
 		return ret;
 
 	task_idx = task->curr_ret_stack;