x86/mm: Enforce ordering constraints for the page alloc lock in the PoD code

x86/mm: Enforce ordering constraints for the page alloc lock in the PoD code

The page alloc lock is sometimes used in the PoD code, with an
explicit expectation of ordering. Use our ordering constructs in the
mm layer to enforce this.

The additional book-keeping variables are kept in the arch_domain
sub-struct, as they are x86-specific to the whole domain.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>