metrics: read /sys/class/block/*/stat correctly

TEST=unit tests, manual tested on panther_embedded
BUG=chromium:415118

Change-Id: I849a9b7849a27d29d9d24bb1575f113e43716bd5
Reviewed-on: https://chromium-review.googlesource.com/218504
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Nathan Bullock <nathanbullock@google.com>
Tested-by: Nathan Bullock <nathanbullock@google.com>
diff --git a/metrics/metrics_daemon.cc b/metrics/metrics_daemon.cc
index b8d013d..5f9d851 100644
--- a/metrics/metrics_daemon.cc
+++ b/metrics/metrics_daemon.cc
@@ -495,7 +495,7 @@
     LOG_IF(WARNING, nchars == sizeof(line))
         << "line too long in " << diskstats_path_;
     line[nchars] = '\0';
-    nitems = sscanf(line, "%*d %*d %" PRIu64 "d %*d %*d %*d %" PRIu64 "d",
+    nitems = sscanf(line, "%*d %*d %" PRIu64 " %*d %*d %*d %" PRIu64,
                     read_sectors, write_sectors);
     if (nitems == 2) {
       success = true;