memory: fix flatview_access_valid RCU read lock/unlock imbalance

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>