nvme: rename nvme_validate_or_alloc_ns to nvme_scan_ns

nvme: rename nvme_validate_or_alloc_ns to nvme_scan_ns

This shorter name much better fits what this function does in
the scanning process.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Javier González <javier.gonz@samsung.com>
Reviewed-by: Joel Granados <j.granados@samsung.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>