nvme: rename nvme_validate_ns to nvme_validate_or_alloc_ns

Use a slightly more descriptive name to enable reusing nvme_validate_ns
in the next patch for a lower level function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>