s390/ftrace: add missing serialization for graph caller patching

CPUs must be serialized also when ftrace_graph_caller gets patched.
This is missing since ftrace function graph support was added on s390.
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/ftrace.c

@@ -268,12 +268,14 @@ NOKPROBE_SYMBOL(prepare_ftrace_return);
 int ftrace_enable_ftrace_graph_caller(void)
 {
 	brcl_disable(ftrace_graph_caller);
+	text_poke_sync_lock();
 	return 0;
 }
 
 int ftrace_disable_ftrace_graph_caller(void)
 {
 	brcl_enable(ftrace_graph_caller);
+	text_poke_sync_lock();
 	return 0;
 }