mnt_ns: Use fdstore to keep mount namespaces

This allows to decrese number of file descriptors,
which are passed to children, and that is need to
close in close_old_files().

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com&gt;:>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>