UBUNTU: SAUCE: shiftfs: record correct creator credentials

UBUNTU: SAUCE: shiftfs: record correct creator credentials

BugLink: https://bugs.launchpad.net/bugs/1872094
When shiftfs is nested we failed to be able to create any files or
access directories because we recorded the wrong creator credentials. We
need to record the credentials of the creator of the lowers mark mount
of shiftfs. Otherwise we aren't privileged wrt to the shiftfs layer in
the nesting case. This is similar to how we always record the user
namespace of the base filesystem.

Suggested-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>