fs_mgr_overlayfs: Remove backing image if failed to allocate scratch

In CreateScratchOnData(), if backing image is allocated but failed to
map, then remove the backing image. Otherwise the unusable (unmappable)
image would waste a lot of space.

Bug: 255593675
Test: On device that don't support mapping gsid scratch,
  run `adb remount` and then check that gsid scratch is removed and
  scratch is allocated on super.
Test: TH run g3-app-compat-main
Change-Id: I0c01c2fd93a30ac0e46f4350c900403337530406