s390/compat: fix compat_sys_statfs() memory corruption

The f_spare field within struct compat_statfs is four bytes larger
than within the native 31 bit struct statfs.
compat_sys_statfs() clears the f_spare field in user space which
means that in compat mode four bytes that are behind the user space
supplied struct compat_statfs will be corrupted (zeroed).

According to Thomas Gleixner's Linux 2.6 history tree this bug is
present since v2.5.74 87880da1 "[PATCH] s390: 31 bit compat.".
So it get's fixed shortly before its 10th anniversary. Tough luck.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/compat.h

@@ -135,7 +135,7 @@ struct compat_statfs {
 	s32		f_namelen;
 	s32		f_frsize;
 	s32		f_flags;
-	s32		f_spare[5];
+	s32		f_spare[4];
 };
 
 #define COMPAT_RLIM_OLD_INFINITY	0x7fffffff