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>