block: Fix AioContext locking in qmp_block_resize()

The AioContext must be unlocked before calling blk_co_unref(), because
it takes the AioContext lock internally in blk_unref_bh(), which is
scheduled in the main thread. If we don't unlock, the AioContext is
locked twice and nested event loops such as in bdrv_graph_wrlock() will
deadlock.

Cc:  <qemu-stable@nongnu.org>
Fixes: https://issues.redhat.com/browse/RHEL-15965
Fixes: 0c7d204
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20231208124352.30295-1-kwolf@redhat.com>

blockdev.c

@@ -2400,8 +2400,9 @@ void coroutine_fn qmp_block_resize(const char *device, const char *node_name,
 
     bdrv_co_lock(bs);
     bdrv_drained_end(bs);
-    blk_co_unref(blk);
     bdrv_co_unlock(bs);
+
+    blk_co_unref(blk);
 }
 
 void qmp_block_stream(const char *job_id, const char *device,