block: convert bdrv_graph_wrlock() to AIO_WAIT_WHILE_UNLOCKED()

The following conversion is safe and does not change behavior:

     GLOBAL_STATE_CODE();
     ...
  -  AIO_WAIT_WHILE(qemu_get_aio_context(), ...);
  +  AIO_WAIT_WHILE_UNLOCKED(NULL, ...);

Since we're in GLOBAL_STATE_CODE(), qemu_get_aio_context() is our home
thread's AioContext. Thus AIO_WAIT_WHILE() does not unlock the
AioContext:

  if (ctx_ && in_aio_context_home_thread(ctx_)) {                \
      while ((cond)) {                                           \
          aio_poll(ctx_, true);                                  \
          waited_ = true;                                        \
      }                                                          \

And that means AIO_WAIT_WHILE_UNLOCKED(NULL, ...) can be substituted.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230309190855.414275-4-stefanha@redhat.com>
Reviewed-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/graph-lock.c

@@ -127,7 +127,7 @@ void bdrv_graph_wrlock(void)
          * reader lock.
          */
         qatomic_set(&has_writer, 0);
-        AIO_WAIT_WHILE(qemu_get_aio_context(), reader_count() >= 1);
+        AIO_WAIT_WHILE_UNLOCKED(NULL, reader_count() >= 1);
         qatomic_set(&has_writer, 1);
 
         /*