From: Dave Hansen <haveblue@us.ibm.com>

In order to turn an 8-way x440 into a 4-way for testing, we often use
mem=(1/2 of total) and maxcpus=4.  maxcpus has always worked, but mem=
hasn't.  The mem= parameter actually changes the kernel's e820 structure,
which manifests itself as max_pfn.  node_end_pfn[] obeys this because of
find_max_pfn_node(), but node_start_pfn[] wasn't modified.

If you have a mem= line that causes memory to stop before the beginning of
a node, you get a condition where start > end (because start was never
modified).  There is a bug check for this, but it was placed just _before_
the error was made :)

Also, the bootmem alloc functions die if you request something of zero size
from them.  This patch avoids that too.  This shouldn't have much of an
effect on non-NUMA systems.