nvme: factor metadata handling out of __nvme_submit_user_cmd

Keep the metadata code in a separate helper instead of making the
main function more complicated.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>