x86: sparsemem: reduce i386 PAE section size

A 1G section size makes memory hotplug too coarse in a virtual
environment.  Retuce it by a factor of 2 to 512M.  I would have liked
to make it smaller, but it runs out of reserved flags in the page flags.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>