net: Call preinit_net() without pernet_ops_rwsem.

When initialising the root netns, we call preinit_net() under
pernet_ops_rwsem.

However, the operations in preinit_net() do not require pernet_ops_rwsem.

Also, we don't hold it for preinit_net() when initialising non-root netns.

To be consistent, let's call preinit_net() without pernet_ops_rwsem in
net_ns_init().

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