x86: sparsemem: reduce i386 PAE section size

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 <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Yasunori Goto <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Dave Hansen <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>