[PATCH v2 0/4] do not use s_dirt in ext4

[Artem Bityutskiy <artem.bityutskiy@linux.intel.com>]

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).

Version 1 of this patch-set can be found here:
https://lkml.org/lkml/2012/3/20/220

Changes between v1 and v2.
  * Rake different strategy - instead of pushing 's_dirt' down "as-is" and
    emulating old behavior, we now just submit the superblock for writing
    straight away, either via the journal or directly. Thank to Jan Kara
    for helping with this.
  * Ted picked some of the patches already, which made this series shorter
    - thanks!
  * This time I've tested the changes using xfstests.
  * Rebased to 3.4-rc1.

Note: Ted, you merged the "mm: export dirty_writeback_interval", but it looks
like we won't need this for ext[23]. However, for other file-systems we will
need this change.

Thanks,
Artem.