cmd/mbf: create report with reasonable perm

Making it inaccessible certainly is not reasonable.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/mbf/main.go

@@ -245,7 +245,7 @@ func main() {
 				if w, err = os.OpenFile(
 					args[0],
 					os.O_CREATE|os.O_EXCL|syscall.O_WRONLY,
-					0,
+					0400,
 				); err != nil {
 					return
 				}