lightnvm: return the correct return value

When memdup_user returns an error, memdup_user has two different return
values, use PTR_ERR to get the correct return value.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Matias Bjørling <matias.bjorling@wdc.com>
Link: https://lore.kernel.org/r/20210413105257.159260-3-matias.bjorling@wdc.com


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