[PATCH] shrink_all_memory(): fix lru_pages handling

At the end of shrink_all_memory() we forget to recalculate lru_pages: it can
be zero.

Fix that up, and add a helper function for this operation too.

Also, recalculate lru_pages each time around the inner loop to get the
balancing correct.

Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>