[PATCH v1 0/9] do not use s_dirt in ext4

This patch-set makes ext4 independent of the VFS superblock management
services. Namely, ext4 does not require to register the 'write_super()' VFS
call-back.

The reason of this exercises is to get rid of the 'sync_supers()' kernel thread
which wakes up every 5 seconds (by default) even if all superblocks are clean.
This is wasteful from power management POW (unnecessary wake-ups).

Note, I tried to optimize 'sync_supers()' instead in 2010, but Al wanted me
to get rid of it instead. See https://lkml.org/lkml/2010/6/6/87
And I think this is right because many file-systems do not need this, for
example btrfs does not use VFS superblock management services at all, so on a
btrfs-based system we currently end-up useless periodic wake-ups source.

Changes for other file-systems are coming later.

The patch-set structure.
1. patches 1,2,3 are independent ext4 cleanups and I ask Ted to merge them as
   soon/long as they are OK. I sent them also independently in order to get
   early comments, but did not get so far, so re-sending.
2. patch 4 exports 'dirty_writeback_interval' and it would be very useful to
   have it merged ASAP to simplify further work
3. patch 5 is also and independent VFS clean-up
4. patches 6-9 actually make ext4 independent on the 'sync_supers()' thread.

Thanks,
Artem.