decnet: fix sparse warnings: context imbalance

Impact: Attribute functions with __acquires(...) resp. __releases(...).

Fix this sparse warnings:
  net/decnet/dn_dev.c:1324:13: warning: context imbalance in 'dn_dev_seq_start' - wrong count at exit
  net/decnet/dn_dev.c:1366:13: warning: context imbalance in 'dn_dev_seq_stop' - unexpected unlock
Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/decnet/dn_dev.c

@@ -1323,6 +1323,7 @@ static inline int is_dn_dev(struct net_device *dev)
 }
 
 static void *dn_dev_seq_start(struct seq_file *seq, loff_t *pos)
+	__acquires(&dev_base_lock)
 {
 	int i;
 	struct net_device *dev;
@@ -1365,6 +1366,7 @@ static void *dn_dev_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 }
 
 static void dn_dev_seq_stop(struct seq_file *seq, void *v)
+	__releases(&dev_base_lock)
 {
 	read_unlock(&dev_base_lock);
 }