We forgot to free new_model_number

Fixes: 013b7ebe ("nvmet: make ctrl model configurable")
Reviewed-by: NChaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: NSagi Grimberg <sagi@grimberg.me>
Signed-off-by: NJens Axboe <axboe@kernel.dk>