Remove usage of hardcoded constants in paging_init().

By chance I spotted a bug in zones_setup involving a change to ZONE_*
constants, due to the ZONE_DMA32 patch from Andi Kleen (which is in -mm).
So, possibly, instead of zones_size[2] you will find zones_size[3] in the
code, but that change is wrong and this patch is still correct.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>