s390/uaccess: fix access_ok compile warnings

On s390 access_ok is a macro which discards all parameters and always
returns 1. This can result in compile warnings which warn about unused
variables like this:

fs/read_write.c: In function 'rw_copy_check_uvector':
fs/read_write.c:684:16: warning: unused variable 'buf' [-Wunused-variable]

Fix this by adding a __range_ok() function which consumes all parameters
but still always returns 1.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/uaccess.h

@@ -50,10 +50,15 @@
 
 #define segment_eq(a,b) ((a).ar4 == (b).ar4)
 
-#define __access_ok(addr, size)	\
-({				\
-	__chk_user_ptr(addr);	\
-	1;			\
+static inline int __range_ok(unsigned long addr, unsigned long size)
+{
+	return 1;
+}
+
+#define __access_ok(addr, size)				\
+({							\
+	__chk_user_ptr(addr);				\
+	__range_ok((unsigned long)(addr), (size));	\
 })
 
 #define access_ok(type, addr, size) __access_ok(addr, size)