x86/mm/p2m: fix pod locking

x86/mm/p2m: fix pod locking

The path p2m-lookup -> p2m-pt->get_entry -> 1GB PoD superpage ->
pod_demand_populate ends in the pod code performing a p2m_set_entry with
no locks held (in order to split the 1GB superpage into 512 2MB ones)

Further, it calls p2m_unlock after that, which will break the spinlock.

This patch attempts to fix that.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>