nvme: split nvme_report_zones

Split multipath support out of nvme_report_zones into a separate helper
and simplify the non-multipath version as a result.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>