net: Add missing annotation for *netlink_seq_start()

Sparse reports a warning at netlink_seq_start()

warning: context imbalance in netlink_seq_start() - wrong count at exit
The root cause is the missing annotation at netlink_seq_start()
Add the missing  __acquires(RCU) annotation

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -2583,6 +2583,7 @@ static void *__netlink_seq_next(struct seq_file *seq)
 }
 
 static void *netlink_seq_start(struct seq_file *seq, loff_t *posp)
+	__acquires(RCU)
 {
 	struct nl_seq_iter *iter = seq->private;
 	void *obj = SEQ_START_TOKEN;