Fail out-of-bounds with a proper status code.

Fixes: d5eff33e ("nvmet: add simple file backed ns support")
Signed-off-by: NSagi Grimberg <sagi@grimberg.me>
Reviewed-by: NChaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>