Let the block device know when sectors can be discarded

[hirofumi@mail.parknet.co.jp: discard _after_ checking for corrupt chains]

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>