Files from such mounts can switch on restore to different files on the
overmounting mount, as we yet don't fully control the mount on which
the file is restored.

v3: make mnt_is_overmounted boolean again
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>