quota: Add support for ->get_nextdqblk() for VFS quota

Add infrastructure for supporting get_nextdqblk() callback for VFS
quotas. Translate the operation into a callback to appropriate
filesystem and consequently to quota format callback.

Signed-off-by: Jan Kara <jack@suse.cz>
diff --git a/fs/reiserfs/super.c b/fs/reiserfs/super.c
index c0306ec..b8f2d1e 100644
--- a/fs/reiserfs/super.c
+++ b/fs/reiserfs/super.c
@@ -802,6 +802,7 @@
 	.write_info = reiserfs_write_info,
 	.alloc_dquot	= dquot_alloc,
 	.destroy_dquot	= dquot_destroy,
+	.get_next_id	= dquot_get_next_id,
 };
 
 static const struct quotactl_ops reiserfs_qctl_operations = {