RFC clustered writeback

Chris,

This is one possible implementation of the clustered writeback idea. It runs OK on ext3 (compiling, syncing, etc.).

The patch is based on 2.6.23-rc3-mm1 and the writeback patches here:
http://lkml.org/lkml/2007/8/19/10

By default, with many dirty inodes, it works as follows:
- store dirty inodes in a radix tree, indexed by their inode numbers
- sweep the whole inode number space in 25s and do it in 5 times
- each time we walk only 1/5 of the inode number space
- pull all inodes with dirty-age larger than 5s to the io dispatching queue

Because it does the work in small batches of 10 inodes, when the system has <=10 dirty inodes, its behavior will reduce to:
- do a full sweep *at once* on every 25s
Which means the disk will flicker once every 25s, not bad :)

The implications for the majority users could be:
- medium-to-heavy writes becomes less seeky
- dirty inodes are getting synced earlier(before: 30s; now: 5-30s)
- less panic for the 'atime' mount option (a future work)

Fengguang