nvme: clean up error handling in nvme_init_ns_head

Use a common label for putting the nshead if needed and only convert
nvme status codes for the one case where it actually is needed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>