On restore we change mounts tree topology and paths in find_remap_mounts
thus mns_is_overmounted does not account remaped overmounts. So to know
which mounts will be actually overmounted after restore we need two much
effort, better prepare these info in advance.

That also should improve performance of mnt_is_overmounted on dump,
as we reuse calculations from previous calls.

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