[PATCH] GFP_ZONETYPES: add commentry on how to calculate

GFP_ZONETYPES define using GFP_ZONEMASK and add commentry

Add commentry explaining the optimisation that we can apply to GFP_ZONETYPES
when the leftmost bit is a 'loaner', it can only be set in isolation.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>