proc: simplify proc_register calling conventions

Return registered entry on success, return NULL on failure and free the
passed in entry.  Also expose it in internal.h as we'll start using it
in proc_net.c soon.

Signed-off-by: Christoph Hellwig <hch@lst.de>
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 0f1692e..488e674 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -162,6 +162,8 @@ extern bool proc_fill_cache(struct file *, struct dir_context *, const char *, i
 /*
  * generic.c
  */
+struct proc_dir_entry *proc_register(struct proc_dir_entry *dir,
+		struct proc_dir_entry *dp);
 extern struct dentry *proc_lookup(struct inode *, struct dentry *, unsigned int);
 struct dentry *proc_lookup_de(struct inode *, struct dentry *, struct proc_dir_entry *);
 extern int proc_readdir(struct file *, struct dir_context *);