Software suspend needs a way of forcing page reclaim, up to the point
where 50% of memory is free.

This patch implements a function to do that:

	int shrink_all_memory(int nr_pages);

Will attempt to reclaim `nr_pages' pages and return them to the free
pages pool.  It returns the number of pages which it actually freed.

If called with a "large" number of pages it will only free up to a few
hundred, so the caller needs to loop on it.

If it returns zero then there is no point in calling it again.