ublk: Switch to memdup_user_nul() helper

Use memdup_user_nul() helper instead of open-coding
to simplify the code.

Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230815114815.1551171-1-ruanjinjie@huawei.com


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