accel/tcg: Honor TLB_DISCARD_WRITE in atomic_mmu_lookup

Using an atomic write or read-write insn on ROM is basically
a happens-never case.  Handle it via stop-the-world, which
will generate non-atomic serial code, where we can correctly
ignore the write while producing the correct read result.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

accel/tcg/cputlb.c

@@ -1831,7 +1831,7 @@ static void *atomic_mmu_lookup(CPUArchState *env, target_ulong addr,
     }
 
     /* Notice an IO access or a needs-MMU-lookup access */
-    if (unlikely(tlb_addr & TLB_MMIO)) {
+    if (unlikely(tlb_addr & (TLB_MMIO | TLB_DISCARD_WRITE))) {
         /* There's really nothing that can be done to
            support this apart from stop-the-world.  */
         goto stop_the_world;