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: 33cf7c90fe2f9 ("net: add real socket cookies")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
index ece31b4..18e5a67 100644
--- a/net/ipv4/syncookies.c
+++ b/net/ipv4/syncookies.c
@@ -346,7 +346,7 @@
 	req->ts_recent		= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 	treq->snt_synack	= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
 	treq->listener		= NULL;
-	ireq->ireq_net		= sock_net(sk);
+	write_pnet(&ireq->ireq_net, sock_net(sk));
 
 	/* We throwed the options of the initial SYN away, so we hope
 	 * the ACK carries the same options again (see RFC1122 4.2.3.8)