x86-64: fix combining of regions in init_memory_mapping()

When nr_range gets decremented, the same slot must be considered for
coalescing with its new successor again.

The issue is apparently pretty benign to native code, but surfaces as a
boot time crash in our forward ported Xen tree (where the page table
setup overall works differently than in native).
Signed-off-by: NJan Beulich <jbeulich@novell.com>
Acked-by: NYinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/mm/init_64.c

@@ -746,7 +746,7 @@ unsigned long __init_refok init_memory_mapping(unsigned long start,
 		old_start = mr[i].start;
 		memmove(&mr[i], &mr[i+1],
 			 (nr_range - 1 - i) * sizeof (struct map_range));
-		mr[i].start = old_start;
+		mr[i--].start = old_start;
 		nr_range--;
 	}