nvme: compare NQN string with right size

Copy subnqns using NVMF_NQN_SIZE as it is < 256

Signed-off-by: Javier González <javier@cnexlabs.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>