PATCH 2.4.0.1.ac10: a KISS memory pressure callback (rev 2)

Attached is a patch against 2.4.0-test1-ac10 which adds a VM pressure
callback list, and cleans up the notifier chain code in the process. 
Since the callback occurs in do_try_to_free_pages, it is rather low
level, giving other kernel code flexibility when it comes to freeing
memory.  Kernel code wishing to handle memory pressure should call
register_freemem_notifier.  Thanks for the suggestions Andi.  Goodnight
all.  :)

-- 
Jeff Garzik              | Liberty is always dangerous, but
Building 1024            | it is the safest thing we have.
MandrakeSoft, Inc.       |      -- Harry Emerson Fosdick