net: move seq_file_single_net to <linux/seq_file_net.h>

This helper deals with single_{open,release}_net internals and thus
belongs here.

Signed-off-by: Christoph Hellwig <hch@lst.de>

include/linux/seq_file_net.h

@@ -28,4 +28,17 @@ static inline struct net *seq_file_net(struct seq_file *seq)
 #endif
 }
 
+/*
+ * This one is needed for single_open_net since net is stored directly in
+ * private not as a struct i.e. seq_file_net can't be used.
+ */
+static inline struct net *seq_file_single_net(struct seq_file *seq)
+{
+#ifdef CONFIG_NET_NS
+	return (struct net *)seq->private;
+#else
+	return &init_net;
+#endif
+}
+
 #endif