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>

exec.c

@@ -3320,7 +3320,6 @@ static bool flatview_access_valid(FlatView *fv, hwaddr addr, int len,
         if (!memory_access_is_direct(mr, is_write)) {
             l = memory_access_size(mr, l, addr);
             if (!memory_region_access_valid(mr, xlat, l, is_write)) {
-                rcu_read_unlock();
                 return false;
             }
         }