dm: ignore discard return value

dm: ignore discard return value

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

For dm-thin change issue_discard() return type to void, in
passdown_double_checking_shared_status() remove the r assignment from
return value of the issue_discard(), for end_discard() hardcode value of
r to 0 that matches only value returned from __blkdev_issue_discard().

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 <ckulkarnilinux@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>