block: avoid potential deadlock during bdrv_graph_wrlock() in bdrv_close()

by passing the BlockDriverState along, so the held AioContext can be
dropped before polling. See commit 31b2ddfea3 ("graph-lock: Unlock the
AioContext while polling") which introduced this functionality for
more information.

The only way to reach bdrv_close() is via bdrv_unref() and for calling
that the BlockDriverState's AioContext lock is supposed to be held.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20231019131936.414246-3-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -5200,7 +5200,7 @@ static void bdrv_close(BlockDriverState *bs)
         bs->drv = NULL;
     }
 
-    bdrv_graph_wrlock(NULL);
+    bdrv_graph_wrlock(bs);
     QLIST_FOREACH_SAFE(child, &bs->children, next, next) {
         bdrv_unref_child(bs, child);
     }