net: Initialise net->passive once in preinit_net().

When initialising the root netns, we set net->passive in setup_net().

However, we do it twice for non-root netns in copy_net_ns() and
setup_net().

This is because we could bypass setup_net() in copy_net_ns() if
down_read_killable() fails.

preinit_net() is a better place to put such an operation.

Let's initialise net->passive in preinit_net().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>