[PATCH] Add SEQ_START_TOKEN #define to seq_file.h

Code that includes seq_file.h uses a magic pointer "(void*)1"
to start a header seq_printf.

This patch adds a #define, so that people can start writing more
readable code.

include/linux/seq_file.h

@@ -65,5 +65,8 @@ int seq_path(struct seq_file *, struct vfsmount *, struct dentry *, char *);
 int single_open(struct file *, int (*)(struct seq_file *, void *), void *);
 int single_release(struct inode *, struct file *);
 int seq_release_private(struct inode *, struct file *);
+
+#define SEQ_START_TOKEN ((void *)1)
+
 #endif
 #endif