Explicitely cast float cookie value to double, fixes erroneous
"RRD was created on another architecture" errors on x86.
Patches provided by Thomas Heil <heil@terminal-consulting.de>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@29874 3c298f89-4303-0410-b956-a3cf2f4a3e73