net/sock: Update sk rcu iterator macro.

Mark hlist node in sk rcu iterator as protected by the rcu.
hlist_next_rcu accomplishes this and silences the warnings
sparse throws.

Found with make C=1 net/ipv4/udp.o on linux-next tag
next-20171009.

Signed-off-by: Tim Hansen <devtimhansen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -737,10 +737,10 @@ static inline void sk_add_bind_node(struct sock *sk,
  *
  */
 #define sk_for_each_entry_offset_rcu(tpos, pos, head, offset)		       \
-	for (pos = rcu_dereference((head)->first);			       \
+	for (pos = rcu_dereference(hlist_first_rcu(head));		       \
 	     pos != NULL &&						       \
 		({ tpos = (typeof(*tpos) *)((void *)pos - offset); 1;});       \
-	     pos = rcu_dereference(pos->next))
+	     pos = rcu_dereference(hlist_next_rcu(pos)))
 
 static inline struct user_namespace *sk_user_ns(struct sock *sk)
 {