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>