RFC Per group orphan list

[Dmitry Monakhov <dmonakhov@openvz.org>]

Currently orphan list is global, which require per-sb mutex
to protect add/del from the list. But it is not really necessary.
Just think about it we have to add inode to orphan list for each
->write_begin call(for each page) if i_size will grow.
For example btrfs support per-ctree orphan list.
So IMHO per-group orphan list seems to be more natural here.
So my proposal is following:
1) Introduce new incompat feature for per-group orphan list. 
2) Store per-group head of the list in group descriptor
   (Where are several reserved fields in grp-desc).
3) Maintain per-group orphan which protected by per-group lst-mutex. 
4) Let fsck to scan group descriptors after unclean umount.Since they
   are close to each other performance penalty will be negligible.

Seems that development complexity of this feature is not too high, 
what do you think.