net: veth -- Don't force to create master peer index if no nsid

In case if master peer of veth device is laying inside
node root net-ns we should not request device index
but rather allow the kernel to number it automatically.

When there is separate net-ns for master peer it should
be safe to request an index though.

Signed-off-by: Cyrill Gorcunov <gorcunov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>