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: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1437391637-31576-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>