x86, mm: Add alloc_low_pages(num)

32bit kmap mapping needs pages to be used for low to high.
At this point those pages are still from pgt_buf_* from BRK, so it is
ok now.
But we want to move early_ioremap_page_table_range_init() out of
init_memory_mapping() and only call it one time later, that will
make page_table_range_init/page_table_kmap_check/alloc_low_page to
use memblock to get page.

memblock allocation for pages are from high to low.
So will get panic from page_table_kmap_check() that has BUG_ON to do
ordering checking.

This patch add alloc_low_pages to make it possible to allocate serveral
pages at first, and hand out pages one by one from low to high.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1353123563-3103-28-git-send-email-yinghai@kernel.org


Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>