If a GFP_NOFS allocation is made when the ZONE_NORMAL inactive list is
full of dirty or under-writeback pages, there is nothing the caller can
do to force some page reclaim.  The caller ends up getting oom-killed.

- In mempool_alloc(), don't try to perform page reclaim again.  Just
  go to sleep and wait for some elements to be returned to the pool.

- In try_to_free_pages(): perform a single, short scan of the LRU and
  if that doesn't work, fail the allocation.  GFP_NOFS allocators know
  how to handle that.