block-backend: fix edge case in bdrv_next_cleanup() where BDS associated to BB changes

block-backend: fix edge case in bdrv_next_cleanup() where BDS associated to BB changes

Same rationale as for commit "block-backend: fix edge case in
bdrv_next() where BDS associated to BB changes". The block graph might
change between the bdrv_next() call and the bdrv_next_cleanup() call,
so it could be that the associated BDS is not the same that was
referenced previously anymore. Instead, rely on bdrv_next() to set
it->bs to the BDS it referenced and unreference that one in any case.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20240322095009.346989-4-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>