writeback: fix time ordering of the per superblock dirty inode lists 6

Recycling the previous changelog:

  When the writeback function is operating in writeback-for-flushing mode
  (as opposed to writeback-for-integrity) and it encounters an I_LOCKed inode,
  it will skip writing that inode.  This is done for throughput and latency:
  move on to another inode rather than blocking for this one.

  Writeback skips this inode by moving it off s_io and onto s_dirty, so that
  writeback can proceed with the other inodes on s_io.

  However that inode movement can corrupt s_dirty's
  reverse-time-orderedness.  Fix that by using the new redirty_tail(), which
  will update the refiled inode's dirtied_when field.

  Note: the behaviour in here is a bit rude: if kupdate happens to come
  across a locked inode then it will defer writeback of that inode for another
  30 seconds.  We'll address that in the next patch.

Address that here.  What we do is to move the skipped inode to the _head_ of
s_dirty, immediately eligible for writeout again.  Instead of deferring that
writeout for another 30 seconds.

One would think that this might cause a livelock: we keep on trying to write
the same locked inode.  But it won't because:

a) if that was the case, it would _already_ be happening on the
   balance_dirty_pages codepath.  Because balance_dirty_pages() doesn't care
   about inode timestamps.

b) if we skipped this inode then we won't have done any writeback.  The
   higher-level writeback paths will see that wbc.nr_to_write didn't change
   and they'll then back off and take a nap.

Cc: Mike Waychison <mikew@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>