xen/mmu: Release just the MFN list, not MFN list and part of pagetables.

We call memblock_reserve for [start of mfn list] -> [PMD aligned end
of mfn list] instead of <start of mfn list> -> <page aligned end of mfn list].

This has the disastrous effect that if at bootup the end of mfn_list is
not PMD aligned we end up returning to memblock parts of the region
past the mfn_list array. And those parts are the PTE tables with
the disastrous effect of seeing this at bootup:

Write protecting the kernel read-only data: 10240k
Freeing unused kernel memory: 1860k freed
Freeing unused kernel memory: 200k freed
(XEN) mm.c:2429:d0 Bad type (saw 1400000000000002 != exp 7000000000000000) for mfn 116a80 (pfn 14e26)
...
(XEN) mm.c:908:d0 Error getting mfn 116a83 (pfn 14e2a) from L1 entry 8000000116a83067 for l1e_owner=0, pg_owner=0
(XEN) mm.c:908:d0 Error getting mfn 4040 (pfn 5555555555555555) from L1 entry 0000000004040601 for l1e_owner=0, pg_owner=0
.. and so on.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/mmu.c

@@ -1227,7 +1227,6 @@ static void __init xen_pagetable_setup_done(pgd_t *base)
 			/* We should be in __ka space. */
 			BUG_ON(xen_start_info->mfn_list < __START_KERNEL_map);
 			addr = xen_start_info->mfn_list;
-			size = PAGE_ALIGN(xen_start_info->nr_pages * sizeof(unsigned long));
 			/* We roundup to the PMD, which means that if anybody at this stage is
 			 * using the __ka address of xen_start_info or xen_start_info->shared_info
 			 * they are in going to crash. Fortunatly we have already revectored
@@ -1235,6 +1234,7 @@ static void __init xen_pagetable_setup_done(pgd_t *base)
 			size = roundup(size, PMD_SIZE);
 			xen_cleanhighmap(addr, addr + size);
 
+			size = PAGE_ALIGN(xen_start_info->nr_pages * sizeof(unsigned long));
 			memblock_free(__pa(xen_start_info->mfn_list), size);
 			/* And revector! Bye bye old array */
 			xen_start_info->mfn_list = new_mfn_list;