inet: prepare inet_base_seq() to run without RTNL

In the following patch, inet_base_seq() will no longer be called
with RTNL held.

Add READ_ONCE()/WRITE_ONCE() annotations in dev_base_seq_inc()
and inet_base_seq().
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -180,8 +180,9 @@ static DECLARE_RWSEM(devnet_rename_sem);
 
 static inline void dev_base_seq_inc(struct net *net)
 {
-	while (++net->dev_base_seq == 0)
-		;
+	unsigned int val = net->dev_base_seq + 1;
+
+	WRITE_ONCE(net->dev_base_seq, val ?: 1);
 }
 
 static inline struct hlist_head *dev_name_hash(struct net *net, const char *name)

net/ipv4/devinet.c

@@ -1837,7 +1837,7 @@ static int in_dev_dump_addr(struct in_device *in_dev, struct sk_buff *skb,
 static u32 inet_base_seq(const struct net *net)
 {
 	u32 res = atomic_read(&net->ipv4.dev_addr_genid) +
-		  net->dev_base_seq;
+		  READ_ONCE(net->dev_base_seq);
 
 	/* Must not return 0 (see nl_dump_check_consistent()).
 	 * Chose a value far away from 0.