nvmet: fix a memory leak

We forgot to free new_model_number

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