overlay: change to normalized, fix error handling

package/gluon-announce/files/lib/gluon/announce/statistics.d/overlay

@@ -2,6 +2,8 @@ local fs = require "nixio.fs"
 local opkg = require "luci.model.ipkg"
 
 local st = fs.statvfs(opkg.overlay_root())
-local used = 100*((st.blocks - st.bfree) / st.blocks) or 0
+local blocks = st.blockss or 0
+local bfree = st.bfree or 0
+local used = ((blocks - bfree) / blocks)
 
 return math.floor(used * 1000 + 0.5) / 1000