nilfs_discard_segment() doesn't wait for completion of discard
requests.  This specifies BLKDEV_IFL_WAIT flag when calling
blkdev_issue_discard() in order to fix the sync failure.
Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: Christoph Hellwig <hch@lst.de>
                                                   start * sects_per_block,
                                                   nblocks * sects_per_block,
                                                   GFP_NOFS,
+                                                  BLKDEV_IFL_WAIT |
                                                   BLKDEV_IFL_BARRIER);
                        if (ret < 0)
                                return ret;
                ret = blkdev_issue_discard(nilfs->ns_bdev,
                                           start * sects_per_block,
                                           nblocks * sects_per_block,
-                                          GFP_NOFS, BLKDEV_IFL_BARRIER);
+                                          GFP_NOFS,
+                                         BLKDEV_IFL_WAIT | BLKDEV_IFL_BARRIER);
        return ret;
 }