ip: constify ip_build_and_send_pkt() socket argument

This function is used to build and send SYNACK packets,
possibly on behalf of unlocked listener socket.

Make sure we did not miss a write by making this socket const.

We no longer can use ip_select_ident() and have to either
set iph->id to 0 or directly call __ip_select_ident()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/ip.h b/include/net/ip.h
index 525dc07..91a6b2c 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -100,7 +100,7 @@
  *	Functions provided by ip.c
  */
 
-int ip_build_and_send_pkt(struct sk_buff *skb, struct sock *sk,
+int ip_build_and_send_pkt(struct sk_buff *skb, const struct sock *sk,
 			  __be32 saddr, __be32 daddr,
 			  struct ip_options_rcu *opt);
 int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt,