xen/mmu: Cleanup xen_pagetable_p2m_copy a bit.

Stefano noticed that the code runs only under 64-bit so
the comments about 32-bit are pointless.

Also we change the condition for xen_revector_p2m_tree
returning the same value (because it could not allocate
a swath of space to put the new P2M in) or it had been
called once already. In such we return early from the
function.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

arch/x86/xen/mmu.c

@@ -1209,9 +1209,11 @@ static void __init xen_pagetable_p2m_copy(void)
 
 	size = PAGE_ALIGN(xen_start_info->nr_pages * sizeof(unsigned long));
 
-	/* On 32-bit, we get zero so this never gets executed. */
 	new_mfn_list = xen_revector_p2m_tree();
-	if (new_mfn_list && new_mfn_list != xen_start_info->mfn_list) {
+	/* No memory or already called. */
+	if (!new_mfn_list || new_mfn_list == xen_start_info->mfn_list)
+		return;
+
 	/* using __ka address and sticking INVALID_P2M_ENTRY! */
 	memset((void *)xen_start_info->mfn_list, 0xff, size);
 
@@ -1229,8 +1231,6 @@ static void __init xen_pagetable_p2m_copy(void)
 	memblock_free(__pa(xen_start_info->mfn_list), size);
 	/* And revector! Bye bye old array */
 	xen_start_info->mfn_list = new_mfn_list;
-	} else
-		return;
 
 	/* At this stage, cleanup_highmap has already cleaned __ka space
 	 * from _brk_limit way up to the max_pfn_mapped (which is the end of