block: Remove unnecessary variable assignment

The parameter "ret" should be zero when running to this line,
no need to set to zero again, remove it.

Signed-off-by: GuoYong Zheng <zhenggy@chinatelecom.cn>
Link: https://lore.kernel.org/r/1642414957-6785-1-git-send-email-zhenggy@chinatelecom.cn


Signed-off-by: Jens Axboe <axboe@kernel.dk>