commit: Replace commit_top_bs on failure after deleting the block job

If there's an error in commit_start() then the block job must be
deleted before replacing commit_top_bs, otherwise it will fail because
of lack of permissions. This happens since the permission system was
introduced in 8dfba279.

Fortunately this bug doesn't seem to be possible to reproduce at the
moment without changing the code.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/commit.c

@@ -374,10 +374,12 @@ fail:
     if (s->top) {
         blk_unref(s->top);
     }
+    job_early_fail(&s->common.job);
+    /* commit_top_bs has to be replaced after deleting the block job,
+     * otherwise this would fail because of lack of permissions. */
     if (commit_top_bs) {
         bdrv_replace_node(commit_top_bs, top, &error_abort);
     }
-    job_early_fail(&s->common.job);
 }