Cleanup __blkdev_issue_discard() a bit:

- remove local variable of 'end_sect'
- remove code block of 'fail'

Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Xiao Ni <xni@redhat.com>
Cc: Mariusz Dabrowski <mariusz.dabrowski@intel.com>
Tested-by: NRui Salvaterra <rsalvaterra@gmail.com>
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>