nvme: check for duplicate identifiers earlier

Lift the check for duplicate identifiers into nvme_init_ns_head, which
avoids pointless error unwinding in case they don't match, and also
matches where we check identifier validity for the multipath case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>