block: Create the commit block job before reopening any image

If the base or overlay images need to be reopened in read-write mode
but the block_job_create() call fails then no one will put those
images back in read-only mode.

We can solve this problem easily by calling block_job_create() first.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Message-id: aa495045770a6f1a7cc5d408397a17c75097fdd8.1464346103.git.berto@igalia.com
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block/commit.c

@@ -236,6 +236,11 @@ void commit_start(BlockDriverState *bs, BlockDriverState *base,
         return;
     }
 
+    s = block_job_create(&commit_job_driver, bs, speed, cb, opaque, errp);
+    if (!s) {
+        return;
+    }
+
     orig_base_flags    = bdrv_get_flags(base);
     orig_overlay_flags = bdrv_get_flags(overlay_bs);
 
@@ -252,16 +257,12 @@ void commit_start(BlockDriverState *bs, BlockDriverState *base,
         bdrv_reopen_multiple(reopen_queue, &local_err);
         if (local_err != NULL) {
             error_propagate(errp, local_err);
+            block_job_unref(&s->common);
             return;
         }
     }
 
 
-    s = block_job_create(&commit_job_driver, bs, speed, cb, opaque, errp);
-    if (!s) {
-        return;
-    }
-
     s->base = blk_new();
     blk_insert_bs(s->base, base);