async: Suppress GCC13 false positive in aio_bh_poll()

GCC13 reports an error :

../util/async.c: In function ‘aio_bh_poll’:
include/qemu/queue.h:303:22: error: storing the address of local variable ‘slice’ in ‘*ctx.bh_slice_list.sqh_last’ [-Werror=dangling-pointer=]
  303 |     (head)->sqh_last = &(elm)->field.sqe_next;                          \
      |     ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
../util/async.c:169:5: note: in expansion of macro ‘QSIMPLEQ_INSERT_TAIL’
  169 |     QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
      |     ^~~~~~~~~~~~~~~~~~~~
../util/async.c:161:17: note: ‘slice’ declared here
  161 |     BHListSlice slice;
      |                 ^~~~~
../util/async.c:161:17: note: ‘ctx’ declared here

But the local variable 'slice' is removed from the global context list
in following loop of the same routine. Add a pragma to silent GCC.

Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20230420202939.1982044-1-clg@kaod.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/async.c

@@ -164,7 +164,21 @@ int aio_bh_poll(AioContext *ctx)
 
     /* Synchronizes with QSLIST_INSERT_HEAD_ATOMIC in aio_bh_enqueue().  */
     QSLIST_MOVE_ATOMIC(&slice.bh_list, &ctx->bh_list);
+
+    /*
+     * GCC13 [-Werror=dangling-pointer=] complains that the local variable
+     * 'slice' is being stored in the global 'ctx->bh_slice_list' but the
+     * list is emptied before this function returns.
+     */
+#if !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpragmas"
+#pragma GCC diagnostic ignored "-Wdangling-pointer="
+#endif
     QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
+#if !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
 
     while ((s = QSIMPLEQ_FIRST(&ctx->bh_slice_list))) {
         QEMUBH *bh;