block: Require .bdrv_child_perm() with child nodes

All block drivers that can have child nodes implement .bdrv_child_perm()
now. Make this officially a requirement by asserting that only drivers
without children can omit .bdrv_child_perm().
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Acked-by: NFam Zheng <famz@redhat.com>

block.c

@@ -1360,8 +1360,9 @@ static int bdrv_check_perm(BlockDriverState *bs, uint64_t cumulative_perms,
                                     cumulative_shared_perms, errp);
     }
 
-    /* Drivers may not have .bdrv_child_perm() */
+    /* Drivers that never have children can omit .bdrv_child_perm() */
     if (!drv->bdrv_child_perm) {
+        assert(QLIST_EMPTY(&bs->children));
         return 0;
     }
 
@@ -1420,8 +1421,9 @@ static void bdrv_set_perm(BlockDriverState *bs, uint64_t cumulative_perms,
         drv->bdrv_set_perm(bs, cumulative_perms, cumulative_shared_perms);
     }
 
-    /* Drivers may not have .bdrv_child_perm() */
+    /* Drivers that never have children can omit .bdrv_child_perm() */
     if (!drv->bdrv_child_perm) {
+        assert(QLIST_EMPTY(&bs->children));
         return;
     }