crash_sender: use more random tmpdir

The POSIX spec often requires 6 X's in temp paths.  Change the code
to use that rather than the non-standard 4.

BUG=chromium:275910
TEST=`cbuildbot {arm,amd64,x86}-generic-full` works
TEST=`crash_sender` on board works

Change-Id: I97f5959afcb9f60dfd379d517904d5bbf5c502e3
Reviewed-on: https://chromium-review.googlesource.com/169680
Reviewed-by: Ben Chan <benchan@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/crash_reporter/crash_sender b/crash_reporter/crash_sender
index 458cb17..a523d98 100755
--- a/crash_reporter/crash_sender
+++ b/crash_reporter/crash_sender
@@ -621,7 +621,7 @@
     fi
   done
 
-  TMP_DIR="$(mktemp -d /tmp/crash_sender.XXXX)"
+  TMP_DIR="$(mktemp -d /tmp/crash_sender.XXXXXX)"
 
   # Send system-wide crashes
   send_crashes "/var/spool/crash"