writeback: merge for_kupdate and !for_kupdate cases

Unify the logic for kupdate and non-kupdate cases.  There won't be
starvation because the inodes requeued into b_more_io will later be
spliced _after_ the remaining inodes in b_io, hence won't stand in the way
of other inodes in the next run.

It avoids unnecessary redirty_tail() calls, hence the update of
i_dirtied_when.  The timestamp update is undesirable because it could
later delay the inode's periodic writeback, or may exclude the inode from
the data integrity sync operation (which checks timestamp to avoid extra
work and livelock).

===
How the redirty_tail() comes about:

It was a long story..  This redirty_tail() was introduced with
wbc.more_io.  The initial patch for more_io actually does not have the
redirty_tail(), and when it's merged, several 100% iowait bug reports
arised:

reiserfs:
        http://lkml.org/lkml/2007/10/23/93

jfs:
        commit 29a424f2
        JFS: clear PAGECACHE_TAG_DIRTY for no-write pages

ext2:
        http://www.spinics.net/linux/lists/linux-ext4/msg04762.html

They are all old bugs hidden in various filesystems that become "visible"
with the more_io patch.  At the time, the ext2 bug is thought to be
"trivial", so not fixed.  Instead the following updated more_io patch with
redirty_tail() is merged:

	http://www.spinics.net/linux/lists/linux-ext4/msg04507.html



This will in general prevent 100% on ext2 and possibly other unknown FS bugs.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Martin Bligh <mbligh@google.com>
Cc: Michael Rubin <mrubin@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>