Fix up the logic which decides when the caller can dip into page reserves.

- If the caller has realtime scheduling policy, or if the caller cannot run
  direct reclaim, then allow the caller to use up to a quarter of the page
  reserves.

- If the caller has __GFP_HIGH then allow the caller to use up to half of
  the page reserves.

- If the caller has PF_MEMALLOC then the caller can use 100% of the page
  reserves.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>