BDS.inherits_from does not always point to an immediate parent node.
When launching a block job with a filter node, for example, the node
directly below the filter will not point to the filter, but keep its old
pointee (above the filter).

If that pointee goes away while the job is still running, the node's
inherits_from will not be updated and thus point to garbage.  To fix
this, bdrv_unref_child() has to check not only the parent node's
immediate children for nodes whose inherits_from needs to be cleared,
but its whole subtree.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190703172813.6868-7-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>