kernel: add overlayfs stacking fix from upstream

Overlayfs stacking was broken in 4.1.12 but now fixed in upstream branch.
Stacking allows experimenting with configurations without the need to
reset a device to factory config.

Signed-off-by: Roman Yeryomin <roman@advem.lv>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 47714