nvme: remove nvme_revalidate_ns

The function is used in two places, and the shared code for those will
diverge later in this series.

Instead factor out a new helper to get the ids for a namespace, simplify
the calling conventions for nvme_identify_ns and just open code the
sequence.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>