memory: fix flatview_access_valid RCU read lock/unlock imbalance
    
    Fixes: 11e732a5ed46903f997985bed4c3767ca28a7eb6
    Reported-by: Cornelia Huck <cohuck@redhat.com>
    Reported-by: luigi burdo <intermediadc@hotmail.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    Tested-by: Cornelia Huck <cohuck@redhat.com>
    Tested-by: Thomas Huth <thuth@redhat.com>
    Message-id: 20180307130238.19358-1-pbonzini@redhat.com
    Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
    (cherry picked from commit b39b61e410022f96ceb53d4381d25cba5126ac44)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>