block: fix permission update in bdrv_replace_node

It's wrong to OR shared permissions. It may lead to crash on further
permission updates.
Also, no needs to consider previously calculated permissions, as at
this point we already bind all new parents and bdrv_get_cumulative_perm
result is enough. So fix the bug by just set permissions by
bdrv_get_cumulative_perm result.

Bug was introduced in long ago 234ac1a9, in 2.9.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190824100740.61635-1-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>