mm/page_alloc.c: use '__paginginit' instead of '__init'

fix section mismatch:

Section mismatch in reference from the function free_area_init_node() to the function .init.text:set_pageblock_order()
The function __meminit free_area_init_node() references
a function __init set_pageblock_order().
If set_pageblock_order is only used by free_area_init_node then
annotate set_pageblock_order with a matching annotation.