net: set a proper network namespace to create a socket

Each socket has to be restored from a proper network namespaces
where it was created.

We set a specified network namespace before restoring a socket.
A task network namespace is set after restoring all files.

v2: don't set the root netns for transport sockets

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