block/copy-before-write: fix permission

In case when source node does not have any parents, the condition still
works as required: backup job do create the parent by

  block_job_create -> block_job_add_bdrv -> bdrv_root_attach_child

Still, in this case checking @perm variable doesn't work, as backup job
creates the root blk with empty permissions (as it rely on CBW filter
to require correct permissions and don't want to create extra
conflicts).

So, we should not check @perm.

The hack may be dropped entirely when transactional insertion of
filter (when we don't try to recalculate permissions in intermediate
state, when filter does conflict with original parent of the source
node) merged (old big series
"[PATCH v5 00/45] Transactional block-graph modifying API"[1] and it's
current in-flight part is "[PATCH v8 0/7] blockdev-replace"[2])

[1] https://patchew.org/QEMU/20220330212902.590099-1-vsementsov@openvz.org/
[2] https://patchew.org/QEMU/20231017184444.932733-1-vsementsov@yandex-team.ru/

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Tested-by: Fiona Ebner <f.ebner@proxmox.com>
Message-Id: <20240313152822.626493-2-vsementsov@yandex-team.ru>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

block/copy-before-write.c

@@ -364,9 +364,13 @@ cbw_child_perm(BlockDriverState *bs, BdrvChild *c, BdrvChildRole role,
                            perm, shared, nperm, nshared);
 
         if (!QLIST_EMPTY(&bs->parents)) {
-            if (perm & BLK_PERM_WRITE) {
-                *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
-            }
+            /*
+             * Note, that source child may be shared with backup job. Backup job
+             * does create own blk parent on copy-before-write node, so this
+             * works even if source node does not have any parents before backup
+             * start
+             */
+            *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
             *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
         }
     }