s390/ftrace: avoid pointer arithmetics with function pointers

Pointer arithmetics with function pointers is not really defined, but
seems to do the right thing. Let's cast to a void pointer to have a
defined behaviour, at least when using gcc.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
index 4c80720..1014ad5 100644
--- a/arch/s390/kernel/ftrace.c
+++ b/arch/s390/kernel/ftrace.c
@@ -178,7 +178,7 @@
 
 	offset = ((void *) prepare_ftrace_return -
 		  (void *) ftrace_graph_caller) / 2;
-	return probe_kernel_write(ftrace_graph_caller + 2,
+	return probe_kernel_write((void *) ftrace_graph_caller + 2,
 				  &offset, sizeof(offset));
 }
 
@@ -186,7 +186,7 @@
 {
 	static unsigned short offset = 0x0002;
 
-	return probe_kernel_write(ftrace_graph_caller + 2,
+	return probe_kernel_write((void *) ftrace_graph_caller + 2,
 				  &offset, sizeof(offset));
 }