Use actual partition size and content offset to calculate it.
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

Backport of r46260

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46261 3c298f89-4303-0410-b956-a3cf2f4a3e73