stage0/gc: try to avoid double overlay mounts
stage0/gc: try to avoid double overlay mounts

Before Linux 4.13, it used to be possible to perform double overlayfs
mounts in place. This now fails at mount() with EBUSY.
We were abusing this feature to unconditionally ensure that the stage1
image overlay mount was present before calling gc entrypoint.
This improves stage0 logic to perform the stage1 image overlay mount
only if it doesn't already exist.

Shashwatsh/rkt,rkt/rkt,alban/rkt,joshix/rkt,lucasallan/rkt,rkt/rkt,alban/rkt,ybubnov/rkt,rkt/rkt,rkt/rkt,coreos/rkt,Shashwatsh/rkt,lucasallan/rkt,kinvolk/rkt,lucasallan/rkt,alban/rkt,kinvolk/rkt,Shashwatsh/rkt,coreos/rkt,kinvolk/rkt,lucasallan/rkt,ybubnov/rkt,ybubnov/rkt,joshix/rkt,kinvolk/rkt,coreos/rkt,joshix/rkt,joshix/rkt