ublk: remove the check of ublk_need_req_ref() from __ublk_check_and_get_req

ublk: remove the check of ublk_need_req_ref() from __ublk_check_and_get_req

__ublk_check_and_get_req() is only called from ublk_check_and_get_req()
and ublk_register_io_buf(), the same check has been covered in the two
calling sites.

So remove the check from __ublk_check_and_get_req().

Suggested-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250429022941.1718671-5-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>