exec: Make memory_region_section_get_iotlb use section AS

Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@xilinx.com>

exec.c

@@ -796,7 +796,7 @@ hwaddr memory_region_section_get_iotlb(CPUArchState *env,
             iotlb |= PHYS_SECTION_ROM;
         }
     } else {
-        iotlb = section - address_space_memory.dispatch->map.sections;
+        iotlb = section - section->address_space->dispatch->map.sections;
         iotlb += xlat;
     }