mm: fix section mismatch warning

The register_page_bootmem_info_node() function needs to be marked __init
in order to avoid a new warning introduced by commit f65e91df ("mm:
use early_pfn_to_nid in register_page_bootmem_info_node").

Otherwise you'll get a warning about how a non-init function calls
early_pfn_to_nid (which is __meminit)

Cc: Yang Shi <yang.shi@linaro.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>