xen: fix UP setup of shared_info

upstream commit: 2e8fe719

We need to set up the shared_info pointer once we've mapped the real
shared_info into its fixmap slot.  That needs to happen once the general
pagetable setup has been done.  Previously, the UP shared_info was set
up one in xen_start_kernel, but that was left pointing to the dummy
shared info.  Unfortunately there's no really good place to do a later
setup of the shared_info in UP, so just do it once the pagetable setup
has been done.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
[chrisw@sous-sol.org: backport to 2.6.24.4]
Signed-off-by: Chris Wright <chrisw@sous-sol.org>