net: fix CONFIG_NET_NS=n compilation

I forgot to use write_pnet() in three locations.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 33cf7c90 ("net: add real socket cookies")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>