blockdev: hold AioContext for bdrv_unref() in external_snapshot_clean()

bdrv_unref() requires the AioContext lock because bdrv_flush() uses
BDRV_POLL_WHILE(), which assumes the AioContext is currently held.  If
BDRV_POLL_WHILE() runs without AioContext held the
pthread_mutex_unlock() call in aio_context_release() fails.

This patch moves bdrv_unref() into the AioContext locked region to solve
the following pthread_mutex_unlock() failure:

  #0  0x00007f566181969b in raise () at /lib64/libc.so.6
  #1  0x00007f566181b3b1 in abort () at /lib64/libc.so.6
  #2  0x00005592cd590458 in error_exit (err=<optimized out>, msg=msg@entry=0x5592cdaf6d60 <__func__.23977> "qemu_mutex_unlock") at util/qemu-thread-posix.c:36
  #3  0x00005592cd96e738 in qemu_mutex_unlock (mutex=mutex@entry=0x5592ce9505e0) at util/qemu-thread-posix.c:96
  #4  0x00005592cd969b69 in aio_context_release (ctx=ctx@entry=0x5592ce950580) at util/async.c:507
  #5  0x00005592cd8ead78 in bdrv_flush (bs=bs@entry=0x5592cfa87210) at block/io.c:2478
  #6  0x00005592cd89df30 in bdrv_close (bs=0x5592cfa87210) at block.c:3207
  #7  0x00005592cd89df30 in bdrv_delete (bs=0x5592cfa87210) at block.c:3395
  #8  0x00005592cd89df30 in bdrv_unref (bs=0x5592cfa87210) at block.c:4418
  #9  0x00005592cd6b7f86 in qmp_transaction (dev_list=<optimized out>, has_props=<optimized out>, props=<optimized out>, errp=errp@entry=0x7ffe4a1fc9d8) at blockdev.c:2308

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20171206144550.22295-2-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

blockdev.c

@@ -1812,8 +1812,8 @@ static void external_snapshot_clean(BlkActionState *common)
                              DO_UPCAST(ExternalSnapshotState, common, common);
     if (state->aio_context) {
         bdrv_drained_end(state->old_bs);
-        aio_context_release(state->aio_context);
         bdrv_unref(state->new_bs);
+        aio_context_release(state->aio_context);
     }
 }