net: allow to dump and restore more than one network namespace

Restore all network namespaces from the root task and then set
a proper namespace for each task after restoring sockets, because
we need to switch network namespaces to restore sockets.

Each socket has to be created in a proper network namespace.

v2: fix a typo bug

Acked-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>