UBUNTU: SAUCE: overlayfs: fix shitfs special-casing

UBUNTU: SAUCE: overlayfs: fix shitfs special-casing

BugLink: https://bugs.launchpad.net/bugs/1876645
When I picked up Andrei's patch I ported it wrong. We need to initialize
realpath before dereferencing it obviously.

Cc: Stefan Bader <stefan.bader@canonical.com>
Cc: Seth Forshee <seth.forshee@canonical.com>
Cc: Kamal Mostafa <kamal@canonical.com>
Cc: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Cc: Andrei Vagin <avagin@gmail.com>
Fixes: 4e1f6efeedae ("UBUNTU: SAUCE: overlayfs: use shiftfs hacks only with shiftfs as underlay")
Link: https://bugs.launchpad.net/bugs/1857257
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>