From: Jan Kara <jack@ucw.cz>

When there are lots of dirty dquots the vfs_quota_sync() is too slow (it has
O(N^2) behaviour).  Attached patch implements list of dirty dquots for each
superblock and quota type.  Using this lists sync is trivially linear.
Attached patch is against 2.6.5 with journalled quota and previous patch for
hash table size.

(Jan had a test which went from 8 minutes to 0.8 seconds...)