commit: Fix error handling

Apparently some kind of mismerge happened in commit 8dfba279, which
broke the error handling without any real reason by removing the
assignment of the return value to ret in a blk_insert_bs() call.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>

block/commit.c

@@ -364,7 +364,7 @@ void commit_start(const char *job_id, BlockDriverState *bs,
 
     /* Required permissions are already taken with block_job_add_bdrv() */
     s->top = blk_new(0, BLK_PERM_ALL);
-    blk_insert_bs(s->top, top, errp);
+    ret = blk_insert_bs(s->top, top, errp);
     if (ret < 0) {
         goto fail;
     }