quota_v2: Implement get_next_id() for V2 quota format

Implement functions to get id of next existing quota structure in quota
file for quota tree based formats and thus for V2 quota format.

Signed-off-by: Jan Kara <jack@suse.cz>
diff --git a/include/linux/dqblk_qtree.h b/include/linux/dqblk_qtree.h
index ff8b553..0de21e9 100644
--- a/include/linux/dqblk_qtree.h
+++ b/include/linux/dqblk_qtree.h
@@ -15,6 +15,7 @@
 #define QTREE_DEL_REWRITE 6
 
 struct dquot;
+struct kqid;
 
 /* Operations */
 struct qtree_fmt_operations {
@@ -52,5 +53,6 @@
 		entries *= epb;
 	return i;
 }
+int qtree_get_next_id(struct qtree_mem_dqinfo *info, struct kqid *qid);
 
 #endif /* _LINUX_DQBLK_QTREE_H */