nvmet: fix a typo in nvmet_file_ns_enable()

Fix a typo in nvmet_file_ns_enable().

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