memblock/nobootmem: allow alloc_bootmem() to take 0 as low limit

The bootmem wrapper with memblock supports top-down now, so we do not need
to set the low limit to __pa(MAX_DMA_ADDRESS).

The logic should be: good to allocate above __pa(MAX_DMA_ADDRESS), but it
is ok if we can not find memory above 16M on system that has a small
amount of RAM.

Signed-off-by: Yinghai LU <yinghai@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Olaf Hering <olaf@aepfle.de>
Cc: Tejun Heo <tj@kernel.org>
Cc: Lucas De Marchi <lucas.demarchi@profusion.mobi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>