commit: Fix use after free in completion
The final bdrv_set_backing_hd() could be working on already freed nodes
because the commit job drops its references (through BlockBackends) to
both overlay_bs and top already a bit earlier.
One way to trigger the bug is hot unplugging a disk for which
blockdev_mark_auto_del() cancels the block job.
Fix this by taking BDS-level references while we're still using the
nodes.
Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>