exec.c: Use atomic_rcu_read() to access dispatch in memory_region_section_get_iotlb()

When accessing the dispatch pointer in an AddressSpace within an RCU
critical section we should always use atomic_rcu_read(). Fix an
access within memory_region_section_get_iotlb() which was incorrectly
doing a direct pointer access.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-Id: <1437391637-31576-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -954,7 +954,10 @@ hwaddr memory_region_section_get_iotlb(CPUState *cpu,
             iotlb |= PHYS_SECTION_ROM;
         }
     } else {
-        iotlb = section - section->address_space->dispatch->map.sections;
+        AddressSpaceDispatch *d;
+
+        d = atomic_rcu_read(&section->address_space->dispatch);
+        iotlb = section - d->map.sections;
         iotlb += xlat;
     }