nvme: move common logic into nvme_update_ns_info

nvme_update_ns_info_generic and nvme_update_ns_info_block share a
fair amount of logic related to not fully supported namespace
formats and updating the multipath information.  Move this logic
into the common caller.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>