UBIFS: use nicer 64-bit math

Instead of using do_div(), use better primitives from
linux/math64.h.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/fs/ubifs/debug.c b/fs/ubifs/debug.c
index 6ecb01a..a2be115 100644
--- a/fs/ubifs/debug.c
+++ b/fs/ubifs/debug.c
@@ -33,6 +33,7 @@
 #include <linux/module.h>
 #include <linux/moduleparam.h>
 #include <linux/debugfs.h>
+#include <linux/math64.h>
 
 #ifdef CONFIG_UBIFS_FS_DEBUG