nvmet: ignore discard return value

nvmet: ignore discard return value

__blkdev_issue_discard() always returns 0, making the error checking
in nvmet_bdev_discard_range() dead code.

Kill the function nvmet_bdev_discard_range() and call
__blkdev_issue_discard() directly from nvmet_bdev_execute_discard(),
since no error handling is needed anymore for __blkdev_issue_discard()
call.

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>