Fix a typo in nvmet_file_ns_enable().

Fixes: d5eff33e ("nvmet: add simple file backed ns support")
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: NSagi Grimberg <sagi@grimberg.e>
Signed-off-by: NChristoph Hellwig <hch@lst.de>