ublk: remove unused req argument from ublk_sub_req_ref()

ublk: remove unused req argument from ublk_sub_req_ref()

Since commit b749965edda8 ("ublk: remove ublk_commit_and_fetch()"),
ublk_sub_req_ref() no longer uses its struct request *req argument.
So drop the argument from ublk_sub_req_ref(), and from
ublk_need_complete_req(), which only passes it to ublk_sub_req_ref().

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Link: https://lore.kernel.org/r/20250715154244.1626810-1-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>