luci-app-statistics: scale up ping_droprate to range 0-100

In original collectd, values of this metric are in range 0-1.  OpenWrt
previously have a custom patch scaling them up to range 0-100.  That
patch has been removed to align with possibly other deployments.

Ref: https://github.com/openwrt/packages/pull/9677
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>