exec: Add missing rcu_read_unlock

rcu_read_unlock was not called if the address_space_access_valid result is
negative.

This caused (at least) a problem when qemu on PPC/E500+TAP failed to terminate
properly and instead got stuck in a deadlock.

Backports commit 662a97d74f9b34cafe9aeb6d96620a97d768a1fa from qemu

qemu/exec.c

@@ -2244,6 +2244,8 @@ bool address_space_access_valid(AddressSpace *as, hwaddr addr, int len, bool is_
         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)) {
+                // Unicorn: commented out
+                //rcu_read_unlock();
                 return false;
             }
         }