Rename overlayfs to overlay

Since Linux 3.18-rc6, overlayfs has been renamed overlay.

This change was introduced by the following commit in linux.git:
ef94b1864d1ed5be54376404bb23d22ed0481feb ovl: rename filesystem type to "overlay"

Signed-off-by: Lénaïc Huard <lhuard@amadeus.com>
+1
I pushed that change.