mpage_writepages() does a lock_page() on pages to be written back, even
when it is being used for page reclaim writeback.

This is normally OK, because the page is unlocked quickly - pages are
unlocked during writeback and nobody should be performing __GFP_FS
allocations inside lock_page().

But it has introduced a ranking problem in ext3:

generic_file_write
->lock_page
  ->ext3_prepare_write
    ->journal_start	(waits for a commit)

versus

ext3_create()
->journal_start()
  ->ext3_new_inode(GFP_KERNEL)
    ->page reclaim
      ->mpage_writepages
        ->lock_page	(locks up, transaction is held open)

Maybe sometime, I'll have to turn mpage_writepages' lock_page into a
trylock if the caller is PF_MEMALLOC.  But for now, let's make ext3's
inside-transaction allocations use GFP_NOFS.  There is only one of them.