nvme: call nvme_identify_ns as the first thing in nvme_alloc_ns_block

Check if the namespace actually exists as the very first thing and don't
bother with any extra work if not.  This should speed up and simplify
the sequential scanning for NVMe 1.0 devices.

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