block/nullb: delete unnecessary memory free

Commit 2984c868(nullb: factor disk parameters) has a typo. The
nullb_device allocation/free is done outside of null_add_dev. The commit
accidentally frees the nullb_device in error code path.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>