overlayfs: search for upstream filesystem name

upstream switched the word from "overlayfs" to "overlay".
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ef94b1864d1ed5be54376404bb23d22ed0481feb

Ping @jfrazelle @tiborvass @unclejack 
Might should be cherry-picked, since 3.18 will release during the docker-1.4.0 cycle

Signed-off-by: Vincent Batts <vbatts@redhat.com>