md: ignore discard return value

md: ignore discard return value

__blkdev_issue_discard() always returns 0, making all error checking at
call sites dead code.

Simplify md to only check !discard_bio by ignoring the
__blkdev_issue_discard() value.

Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>