mount: do remaps for child-overmount of another overmount

In case we have mounts:

1 /mnt/
2 /mnt/a with parent 1
3 /mnt/a/b with parent 1
4 /mnt/a with parent 2

We determine 2 as needing remap with does_mnt_overmount() and remap it.
Next we mount 4 on top of 2. Next in fixup_remap_mounts() we want to
move 2 back to it's parent 1, but instead move 4 there. So in these case
children-overmounts need to be remapped too.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>