block: Silence Coverity in bdrv_drop_intermediate()

Coverity doesn't like that the return value of bdrv_check_update_perm()
stays unused only in this place (CID 1399710).

Even if checking local_err should be equivalent to checking ret < 0,
let's switch to using the return value to be more consistent (and in
case of a bug somewhere down the call chain, forgetting to assign errp
is more likely than returning 0 for an error case).
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>

block.c

@@ -4350,11 +4350,10 @@ int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
     QLIST_FOREACH_SAFE(c, &top->parents, next_parent, next) {
         /* Check whether we are allowed to switch c from top to base */
         GSList *ignore_children = g_slist_prepend(NULL, c);
-        bdrv_check_update_perm(base, NULL, c->perm, c->shared_perm,
-                               ignore_children, &local_err);
+        ret = bdrv_check_update_perm(base, NULL, c->perm, c->shared_perm,
+                                     ignore_children, &local_err);
         g_slist_free(ignore_children);
-        if (local_err) {
-            ret = -EPERM;
+        if (ret < 0) {
             error_report_err(local_err);
             goto exit;
         }