overlay: ignore EINVAL for cleanup unmount

ignore EINVAL for unmount in the cleanup function as it means the
directory is not a mount point.  It can happen if the cleanup happens
before the mount point is created.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

drivers/overlay/overlay.go

@@ -1475,10 +1475,14 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
 		if retErr != nil {
 			if c := d.ctr.Decrement(mergedDir); c <= 0 {
 				if mntErr := unix.Unmount(mergedDir, 0); mntErr != nil {
+					// Ignore EINVAL, it means the directory is not a mount point and it can happen
+					// if the current function fails before the mount point is created.
+					if !errors.Is(mntErr, unix.EINVAL) {
 						logrus.Errorf("Unmounting %v: %v", mergedDir, mntErr)
 					}
 				}
 			}
+		}
 	}()
 
 	readWrite := !inAdditionalStore