files: open files on overmounted mounts are not supported

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>