x86: cleanup setup_node_zones called by paging_init()

setup_node_zones() calcuates some variables but only use them when
FLAT_NODE_MEM_MAP is set

so change the MACRO postion to avoid calculating.

also change it to static, and rename it to flat_setup_node_zones().

Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>