mm: Add alloc_bootmem_low_pages_nopanic()

We don't need to panic in some case, like for swiotlb preallocating.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1359058816-7615-35-git-send-email-yinghai@kernel.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>