nvme: add nvme_opcode_str function for all nvme cmd types

nvme_opcode_str will handle io/admin/fabrics ops

This improves NVMe errors logging

Signed-off-by: Amit Engel <Amit.Engel@dell.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>